INTRODUCTION  TO  THE  STUDY  OF 
MINERALS  AND  ROCKS 


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Microphotographs  of  snow  crystals.     (After  Bentley.} 


(Frontispiece. ) 


INTRODUCTION 
TO  THE  STUDY  OF  - 

MINEEALS  AND  ROCKS 

A  COMBINED  TEXT-BOOK  AND 
POCKET  MANUAL 


BY 
AUSTIN  FLINT  ROGERS, '  jy."  J>,^%  ,,  ^.  * . 

PROFESSOR  OF  MINERALOGY,   STANF(\RK,  Vll/V^pSIfi,;       ^  r       '     '.    i       ;    « vl 

FELLOW,  GEOLOGICAL  SOCIETY  OF  AMERICA;  FELLOW,  MINERALOGICAL  SOCIETY  OF  AMERICA; 

MEMBER,  MINERALOGICAL  SOCIETY  OF  GREAT  BRITAIN  ;  ASSOCIATE  EDITOR,  AMERICAN 

MINERALOGIST;  FELLOW,  AMERICAN  ACADEMY  OF  ARTS  AND  SCIENCES 


SECOND  EDITION 
SECOND  IMPRESSION 


McGRAW-HILL  BOOK  COMPANY,  INC. 
NEW  YORK:  370  SEVENTH  AVENUE 

LONDON :  6  &  8  BOUVERIE  ST.,  E.  C.  4 
1921 


^GOBYRIGHT,   1912,    1921,  BY  THE 

Mc<sU*Aw-HiLL  BOOK  COMPANY,  INC. 


MAPI.E     FRBSS     YORK    P-4. 


PREFACE 

This  work  is  intended  primarily  as  a  text-book  for  a  year's 
work  in  the  study  of  minerals  and  rocks.  While  full  enough  for 
class  work  it  is  condensed  enough  for  field  work.  There  is  decided 
advantage  in  using  the  same  book  in  the  field  as  in  the  class  room 
and  laboratory. 

Part  II  contains  the  description  of  175  minerals.  These  have 
been  selected  with  great  care,  although  the  list  is  necessarily 
an  arbitrary  one  based  upon  the  author's  experience  and  judg- 
ment. These  include  all  the  common  minerals  and  most  of 
those  of  any  special  economic,  geological,  or  scientific  import- 
ance. Some  of  the  minerals  are  comparatively  rare  but  are 
selected  so  as  to  give  a  comprehensive  view  of  the  mineral 
kingdom  as  a  whole. 

About  a  third  of  the  more  common  and  important  of  the 
minerals  are  distinguished  by  larger  type  than  the  others.  These 
fifty-six  minerals  include  all  the  very  common  minerals  taken 
the  world  over.  In  a  short  course  in  mineralogy  attention 
may  be  confined  exclusively  to  the  shorter  list  and  in  that  case 
other  portions  of  the  book  also  would  have  to  be  disregarded. 

The  section  dealing  with  the  chemical  properties  of  minerals 
has  been  placed  first  because  in  elementary  work  it  is  of  prime 
importance.  A  qualitative  scheme,  especially  applicable  to  min- 
erals in  that  calcium  phosphate,  fluorid,  and  borate  are  provided 
for,  has  been  included. 

In  geometrical  crystallography,  symmetry  has  been  emphasized 
and  the  idea  of  hemihedrism  is  abandoned.  A  tabulation  of 
the  thirty-two  crystal  classes  is  given  but  only  eleven  of  the 
thirty-two  classes  are  described  in  detail.  From  one  to  four 
common  minerals  of  each  class  are  used  as  illustrations.  Groth's 

vii 

r^  *7  I  &  /i  O 


viii  PREFACE 

names  of  the  thirty-two  classes  and  Fedorov's  names  of  forms 
are  used.  The  Miller  symbols  have  been  used  to  the  exclusion 
of  all  others.  Plans  and  elevations  of  crystals  are  made  much 
use  of,  for  they  furnish  a  convenient  means  of  determining  indices 
and  axial  ratios  graphically.  The  section  on  the  internal  struc- 
ture of  crystals  has  been  revised  and  augmented  in  the  light  of 
the  work  done  by  the  Braggs  and  others  in  recent  years. 

Among  the  physical  properties  the  optical  properties  are 
treated  at  some  length  because  of  their  great  value  in  the  deter- 
mination of  minerals.  The  determination  of  the  microscopic  and 
optical  properties  of  minerals  in  crushed  fragments  is  emphasized. 
No  one  is  properly  equipped  to  determine  minerals  until  he 
understands  fairly  well  the  subject  of  optical  crystallography. 

In  Part  II  the  order  of  the  minerals  is  practically  the  same  as 
that  of  Dana's  System  of  Mineralogy  except  that  the  silicates 
are  placed  last.  The  silicates  are  the  most  difficult  minerals 
and  so  they  are  reached  after  the  student  has  gained  consider- 
able experience  with  the  other  groups.  Some  changes  in 
the  standard  nomenclature  of  minerals  may  be  mentioned. 
Bauxite  is  treated  as  a  rock  made  up  of  the  amorphous  mineral 
cliachite  and  the  crystalline  mineral  gibbsite.  Serpentine  is 
also  considered  a  rock,  its  principal  mineral  being  antigorite. 
Collophane,  an  amorphous  calcium  carbonophosphate  is  regarded 
as  the  chief  constituent  of  phosphorite  or  so-called  phosphate 
rock.  It  is  also  the  mineral  of  fossil  bones  and  so  is  a  very  widely 
distributed  mineral.  The  amorphous  minerals  turyite,  cliachite, 
cellophane,  and  halloysite  are  the  amorphous  equivalents  of 
the  following  crystalline  minerals  respectively,  hematite,  gibb- 
site, dahllite,  and  kaolinite.  Natural  glass  and  hydrocarbons 
are  treated  in  an  appendix  to  the  minerals  under  the  term 
miner  aloid. 

In  Part  III  there  is  an  elementary  discussion  of  the  occurrence, 
association,  and  origin  of  minerals.  This  includes  a  brief  descrip- 
tion of  some  of  the  more  common  and  important  rock  types  and 
also  of  the  prominent  classes  of  veins  and  replacement  deposits. 


PREFACE  ix 

Appended  to  Part  IV  there  are  two  tables  for  the  determination 
of  minerals.  In  the  first  the  principal  dependence  is  upon 
crystal  form  and  physical  properties  and  so  it  may  be  used  in 
the  field  as  well  as  in  class  room.  In  using  Table  II  the  mineral 
is  determined  mainly  by  finding  its  constituent  acid  radicals  and 
metals  by  chemical  and  blowpipe  tests  and  so  its  use  is  confined 
to  the  laboratory.  If  a  polarizing  microscope  is  available, 
optical  and  microscopic  tests  will  greatly  aid  in  the  identification 
of  minerals.  The  table  on  page  192  may  be  used  to  advantage. 

A  glossary  of  terms  not  explained  in  the  text  has  been  included 
with  the  index.  Synonyms  and  varieties  are  usually  given  in 
the  glossary  rather  than  in  the  text. 

Thanks  are  due  my  colleague,  Professor  C.  F.  Tolman,  Jr., 
for  suggestions  concerning  certain  portions  of  Part  III.  I  am 
also  indebted  to  my  daughter,  Gene  vie  ve  Rogers,  for  assistance 
in  reading  proof. 

A.  F.  R. 

STANFORD  UNIVERSITY. 


CONTENTS 

PAGE 

PREFACE vii 

SELECT  BIBLIOGRAPHY xv 

INTRODUCTION 1 

PART  I 
THE  PROPERTIES  OF  MINERALS 

THE  CHEMICAL  PROPERTIES: 

A.  Chemical  Principles: 

1.  Elements 5 

2.  Chemical  Compounds 8 

3.  Derivation  of  Chemical  Formulae 11 

4.  Variations  in  the  Chemical  Composition  of  Minerals.    ...  13 

5.  Isomorphism 14 

6.  Solid  Solutions  not  Due  to  Isomorphous  Mixtures 16 

7.  Minerals  of  Colloidal  Origin 17 

8.  Polymorphism 18 

B.  Blowpipe  Analysis: 

1.  Apparatus  used  in  Blowpipe  Analysis 20 

2.  Reagents  used  in  Blowpipe  Analysis 22 

3.  The  Operations  of  Blowpipe  Analysis 25 

4.  Select  Blowpipe  and  Wet  Tests 40 

THE  MORPHOLOGICAL  PROPERTIES: 

1.  The  Amorphous  Condition 54 

2.  The  General  Properties  of  Crystals 55 

3.  The  Measurement  of  Crystals 56 

4.  The  Symmetry  of  Crystals 60 

5.  The  Forms  of  Crystals 68 

6.  The  Notation  of  Crystal  Faces 73 

7.  The  Classification  of  Crystals. 78 

8.  Crystal  Drawing 83 

9.  The  Isometric  System .    .  89 

10.  The  Tetragonal  System .97 

11.  The  Hexagonal  System 102 

12.  The  Orthorhombic  System ...  Ill 

13.  The  Monoclinic  System 115 

xi 


xil  CONTENTS 

PAGE 

14.  The  Triclinic  System 121 

15.  Composite  Crystals  and  Crystalline  Aggregates 123 

16.  Cleavage  and  Parting 129 

17.  The  Internal  Structure  of  Crystals 132 

THE  PHYSICAL  PROPERTIES: 

A.  The  Simpler  Physical  Properties: 

1.  Specific  Gravity 148 

2.  Hardness 152 

3.  Luster 154 

4.  Color 154 

5.  Streak  .    .    .    , 155 

B.  The  Optical  Properties: 

1.  The  Nature  of  Light 156 

2.  Refraction  of  Light 159 

3.  Polarized  Light 167 

4.  Double  Refraction 169 

5.  The  Nicol  Prism 171 

6.  The  Polarizing  Microscope 172 

7.  Interference  Colors .  175 

8.  Vibration  or  Extinction  Directions 184 

9.  The  Determination  of  the  Indices  of  Refraction  in  Doubly- 

Refracting  Crystals 185 

10.  Direction  of  the  Faster  and  Slower  Ray 187 

11.  Classification  of  Crystals  from  an  Optical  Standpoint.    .    .189 

12.  Interference  Figures 193 

13.  Optical  Properties  of  Twin-crystals.    .    v 200 

14.  Absorption  and  Pleochroism 201 

15.  Suggested  Outline  of  Tests  to  Illustrate  the  Optical  Prop- 

erties of  Minerals 204 

16.  List  of   Minerals   Arranged   According  to  the   Indices  of 

Refraction 207 

PART  II 

THE  DESCRIPTION  OF  IMPORTANT  MINERALS  AND 
MINERALOIDS 

Introductory 211 

A.  Minerals: 

1.  Elements 213 

2.  Sulfids ,   .'V  ...  225 

3.  Sulfo-salts.                                   243 


CONTENTS  xiii 

PAGE 

4.  Haloids 251 

5.  Oxids 258 

6.  Aluminates,  Ferrites,  etc 277 

7.  Hydroxids 284 

8.  Carbonates 290 

9.  Phosphates,  Nitrates,  Borates,  etc 310 

10.  Sulfates .  326 

11.  Tungstates  and  Molybdates.    .  ' 338 

12.  Silicates 341 

B.  Mineraloids. 

(Glass  and  Hydrocarbons) 413 

PART  III 

THE  OCCURRENCE,  ASSOCIATION,  AND   ORIGIN    OF  MINERALS 

A.  General  Principles 417 

1.  Association  of  Minerals 417 

2.  Order  of  Succession 418 

3.  Processes  of  Mineral  Formation 418 

4.  Synthesis  of  Minerals 418 

5.  Alteration  and  Replacement  of  Minerals 420 

B.  Mineral  Occurrences 422 

1.  Igneous  Rocks 422 

(a)  General  Discussion 422 

(6)  Granite-Rhyolite  Series.    .    . 429 

(c)  Syenite-Trachyte  Series 431 

(d)  Granodiorite-Dacite  Series 432 

(e)  Diorite-Andesite  Series 432 

(/)  Gabbro-Auganite  Series 433 

(0)  Olivine  Gabbro-Basalt  Series 434 

(h)  Peridotite-Limburgite  Series 436 

(1)  Other  Feldspar-Free  Igneous  Rocks 437 

2.  Volcanic  Emanations 437 

3.  Pegmatites 437 

4.  Pyroclastic  Rocks 438 

5.  Sedimentary  Rocks 439 

(a)  of  Mechanical  Origin 439 

(6)  of  Organic  Origin 442 

(c)  of  Chemical  Origin 445 

6.  Metamorphic  Rocks 450 

(a)  Regional  Metamorphism 451 


XIV  CONTENTS 

PAGE 

(6)  Contact  Metamorphism 453 

(c)  Hydrothermal  Metamorphism.    . 454 

7.  Veins  and  Replacement  Deposits ...  455 

(a)  High- Temperature  Deposits 456 

(6)  Intermediate-Temperature  Deposits 457 

(c)  Low-Temperature  Deposits 457 

(d)  Stages  in  Mineral  Formation 458 

(e)  Ore-deposits  riot  Related  to  Igneous  Intrusions  .    .    .  458 

(/)  Zone  of  Oxidation 459 

(0)  Supergene  Enrichment 460 

PART  IV 

THE  DETERMINATION  OF  MINERALS 

INTRODUCTION 463 

REMARKS  ON  THE  USE  OF  TABLE  I 464 

REMARKS  ON  THE  USE  OF  TABLE  II 464 

THE  DETERMINATION  OF  MINERALS  BY  OPTICAL  TESTS 465 

TABLE  I .    .  466 

TABLE  II 492 

INDEX  AND  GLOSSARY.                                                                                .  505 


SELECT  BIBLIOGRAPHY 

PART  I 

THE  PROPERTIES  OF  MINERALS 
THE  CHEMICAL  PROPERTIES  OP  MINERALS 

ARZRUNI:  Physikalische  Chemie  der  Krystalle.     Vieweg  u.   Sohn,    Braun- 
schweig, 1893. 

BRAUNS:  Chemische  Mineralogie.     Tauchnitz,  Leipzig,   1896. 
BRUSH    (PENPIELD):  Manual   of  Determinative   Mineralogy.     Wiley,     New 

York,  1906  (16th  edition). 

DOELTER:  Physikalisch-Chemische  Mineralogie.     Joh.  Barth,  Leipzig,  1905. 
DOELTER:  Handbuch  der  Mineralchemie.    I.  Steinkopf,    Dresden,    1912 — 

Band  I  (Band  II,  III,  IV  not  complete). 

GROTH:  Chemische  Krystallographie.     Engelmann,  Leipzig,  1906  (4  vols.). 
GROTH    (MARSHALL):  Introduction    to    Chemical     Crystallography.     Wiley, 

New  York,  1906. 
MELLOR:  Modern     Inorganic     Chemistry.     Longmans,     Green     and     Co., 

London,  1917  (2d  edition). 
NOTES,  A.  A. :  Qualitative  Chemical  Analysis.     Macmillan,  New  York,  1920 

(8th  edition). 
PLATTNER    (KOLBECK)  :  Probierkunst    mil    dem   Lotrohre.     Johann    Barth, 

Leipzig,  1907  (7th  edition). 
PRESCOTT  AND  JOHNSON:  Qualitative  Chemical  Analysis.     Van  Nostrand, 

New  York,  1908  (6th  edition). 

THE  MORPHOLOGICAL  PROPERTIES  OF  MINERALS 

BAUERMANN:  Text-book     of    Systematic     Mineralogy.     Longmans,     Green 
and  Co.,  London,  1889. 

BRAGG    AND    BRAGG:  X-rays   and   Crystal  Structure.     G.    Bell   and   Sons, 
London,  1918  (3rd  edition). 

FRIEDEL:  Lecons  de  Cristallographie.    Hermann  et  Fils,  Paris,  1911. 

GROTH:  Physikalische     Krystallographie.     Wilhelm     Engelmann,     Leipzig, 
1905  (4th  edition). 

HILTON:  Mathematical  Crystallography  and  the  Theory  of  Groups  of  Move- 
ments.    Clarendon  Press,  Oxford,  1903. 
xv 


xvi  SELECT  BIBLIOGRAPHY 

LEWIS:  A  Treatise  on  Crystallography.     Cambridge  University  Press,  1899. 
LIEBISCH:  Grundriss  der  Physikalischen  Krystallographie.     Veit  and   Co., 

Leipzig,  1896. 

MOSES:  Characters  of  Crystals.     Van  Nostrand,  New  York,  1899. 
REEKS:  Hints  for  Crystal  Drawing.     Longmans,  Green  and  Co.,  London, 

1908. 

SCHOENFLIES:  Krystallsysteme  und  Krystallstructur.     Teubner,  Leipzig,  1891. 
STORY- MASKELYNE:  Crystallography.     Clarendon  Press,  Oxford,  1895. 
TUTTON:  Crystallography  and  Practical  Crystal  Measurement.     Macmillan, 

London,  1911. 

WALKER:  Crystallography.     McGraw-Hill  Book  Co.,  New  York,  1914. 
WILLIAMS:  Elements  of  Crystallography.     Henry  Holt  and  Co.,  New  York, 

1902  (3d  edition). 

THE  PHYSICAL  PROPERTIES  OP  MINERALS 

GROTH:  Physikalische  Krystallographie  (see  above). 

LIEBISCH:  Grundriss  der  Physikalischen  Krystallographie  (see  above). 

MOSES:  Characters  of  Crystals  (see  above). 

VOIGT:  Lehrbuch  der  Kristallphysik.     Teubner,  Leipzig,  1910. 

THE  OPTICAL  PROPERTIES  OF  MINERALS 

DUPARC  ET  PEARCE:  Traite  de  Technique  Mineralogique  et  Petrographique. 

(Premiere  Partie.)     Veit  u.  Co.,  Leipzig,  1907. 
GROTH  (JACKSON):  The  Optical  Properties  of  Crystals.     Wiley,  New  York, 

1910. 

IDDINGS:  Rock  Minerals.     Wiley,  New  York,  1911  (2d  edition). 
JOHANNSEN:  Manual  of  Petrographic  Methods.     McGraw-Hill,  New  York, 

1914. 

LIEBISCH:  Grundriss  der  Physikalischen  Krystallographie  (see  above). 
LUQUER:  Minerals   in  Rock   Sections.     Van    Nostrand,    New   York,    1913 

(4th  edition). 

MOSES:  Characters  of  Crystals  (see  above). 
RINNE:  Elementare    Anleitung   Zu   Kristallographisch-optischen    Untersuch- 

ungen;  Max  Janecke,  Leipzig,  1912. 

TUTTON:  Crystallography  and  Practical  Crystal  Measurement  (see  above). 
WEINSCHENK  (CLARK):  Petrographic  Methods.     McGraw-Hill,   New  York, 

1912. 
WINCHELL  AND  WiNCHELL:  Elements  of  Optical  Mineralogy.     Van  Nostrand, 

New  York,  1909. 
WRIGHT:  The    Methods    of    Petrographic-M icroscopic    Research.     Carnegie 

Institution,  Washington,  1911. 


SELECT  BIBLIOGRAPHY  xvii 

PART  II 
THE  DESCRIPTION  OF  IMPORTANT  MINERALS 

BAYLEY:  Descriptive  Mineralogy.     D.  Appleton  and  Co.,  New  York,  1917. 

BRAUNS  (SPENCER):  The  Mineral  Kingdom.  J.  F.  Schreiber,  Esslingen, 
1912. 

DANA:  System  of  Mineralogy.  Wiley,  New  York,  1892  (6th  edition). 
1st  appendix,  1899.  2d  appendix,  1909.  3d  appendix,  1915. 

GROTH:  Tabellarische  Uebersicht  der  Mineralien.  Vieweg  u.  Sohn,  Braun- 
schweig, 1898  (4th  edition). 

FOOTE:  Complete  Mineral  Catalog.     Foote  Mineral  Co.,  Philadelphia,  1909. 

HINTZE:  Handbuch  der  Mineralogie.  Veit  u.  Co.,  Leipzig,  1897  II  Band. 
(I  Band,  not  complete). 

MIERS:  Mineralogy.     Macmillan,  London,  1902. 

NAUMANN  (ZIRKEL):  Elemente  der  Mineralogie.  Engelmann,  Leipzig,  1901 
(14th  edition). 

ROSENBUSCH  (WULFING):  Mikroskopische  Physiographic  der  petrographisch 
wichtigen  Mineralien.  E.  Schweizerbartsche  Verlagshandlung, 
Stuttgart,  1905. 

PART  III 

THE  OCCURRENCE,  ASSOCIATION,  AND  ORIGIN  OF  MINERALS 

BECK  (WEED):  The  Nature  of  Ore  Deposits.  McGraw-Hill,  New  York, 
1911  (2d  edition). 

BEYSCHLAG,  KRUSCH  AND  VOGT  (TRUSCOTT):  The  Deposits  of  the  Useful, 
Minerals  and  Rocks.  Macmillan,  London,  1914  (3  vols.). 

BRAUNS:  Chemische  Mineralogie  (see  above). 

CLARKE  :  The  Data  of  Geochemistry.  U.  S.  Geological  Survey,  Washington, 
1920  Bulletin  695  (4th  edition). 

DALY:  Igneous  Rocks  and  their  Origin.     McGraw-Hill,  New  York,  1914. 

GEIKIE,  JAMES:  Structural  and  Field  Geology.  Oliver  and  Boyd,  Edin- 
burgh, 1908  (2d  edition). 

FARRINGTON:  Meteorites.     O.  C.  Farrington,  Chicago,  1915. 

GRABAU:  Principles  of  Stratigraphy.     A.  G.  Seiler  and  Co.,  New  York,  1913. 

IDDINGS:  Igneous  Rocks.     Wiley,  New  York,  1909  (2  vols.). 

KEMP:  Handbook  of  Rocks.     Van  Nostrand,  New  York,  1911  (5th  edition). 

KLOCKMANN:  Lehrbuch  der  Mineralogie.  Ferd.  Enke,  Stuttgart,  1907 
(4th  edition). 

LACROIX:  Mineralogie  de  la  France  et  de  ses  Colonies.  Librairie  Poly- 
technique,  Paris,  1893-1910  (4  vols.). 


xviii  SELECT  BIBLIOGRAPHY 

LBITH  AND  MEAD:  Metamorphic  Geology.  Henry  Holt  and  Co.,  New  York, 
1915. 

LINDGREN:  Mineral  Deposits.     McGraw-Hill,  New  York,  1919  (2d  edition). 

MEUNIEB:  Les  Methodes  de  Synthese  en  Mineralogie.  Librairie  Poly- 
technique,  Paris,  1891. 

MOSES  AND  PARSONS:  Mineralogy,  Crystallography  and  Blowpipe  Analyses, 
Van  Nostrand,  New  York,  1916  (5th  edition). 

PIRSSON:  Rocks  and  Rock-Minerals.     Wiley,  New  York,  1908. 

WEINSCHENK  (JOHANNSEN):  The  Fundamental  Principles  of  Petrology. 
McGraw-Hill,  New  York,  1916. 

PART  IV 
THE  DETERMINATION  OF  MINERALS 

BRUSH  (PENFIELD):  Manual  of  Determinative  Mineralogy  (see  above). 
KRAUS  AND  HUNT:  Mineralogy.     McGraw-Hill,  New  York,  1920. 


INTRODUCTION  TO  THE  STUDY  OF 
MINERALS  AND  ROCKS 


INTRODUCTION 

A  mineral  may  be  defined  as  a  naturally  occurring  homo- 
geneous, inorganic  substance  of  definite  or  fairly  definite  chem- 
ical composition  and  with  characteristic  physical  properties. 
Minerals  are  to  a  large  extent  the  units  which  make  up  the  rocks 
of  the  earth's  crust  or  outer  shell.  Many  of  them  are  useful  to 
man.  A  knowledge  of  minerals  is  obviously  necessary  to  the 
geologist  and  to  some  extent  to  the  engineer,  the  chemist,  and 
the  metallurgist. 

About  a  thousand  definite  minerals  are  well  established  but 
many  of  them  are  very  rare  and  have  been  found  at  but  a  single 
locality.  Only  about  two  hundred  or  so  are  of  much  importance 
from  either  a  geological  or  economic  standpoint. 

The  information  concerning  minerals  may  be  conveniently 
placed  under  two  general  heads:  (1)  the  properties  of  minerals 
and  (2)  the  relations  of  associated  minerals. 

The  properties  of  minerals  naturally  divide  themselves  in  three 
groups:  (1)  chemical,  (2)  morphological,  and  (3)  physical.  The 
morphological  properties,  or  those  concerned  with  external 
form  and  internal  structure,  form  a  connecting  link  between  the 
other  two. 

The  most  fundamental  fact  about  a  mineral  is  its  chemical 
composition.  Many  minerals  have  a  definite  chemical  composi- 
tion; for  all  minerals  it  is  definite  within  certain  limits.  The 
classification  of  minerals  is  primarily  a  chemical  one.  For 
sight  recognition  the  morphological  and  physical  properties  are 
very  important,  but  for  the  purposes  of  exact  identification  it  is 
often  necessary  and  always,  advisable  to  make  a  more  or  less 

1 


2  INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

complete  chemical  examination  of  the  mineral.  Wet  tests 
must  often  be  resorted  to,  but  the  blowpipe  is  very  useful  in  the 
determination  of  the  chemical  composition  of  a  mineral.  For 
these  reasons  inorganic  chemistry  is  the  important  prerequisite 
for  the  study  of  mineralogy. 

The  great  majority  of  minerals  when  produced  under  favorable 
conditions  assume  the  geometric  forms  called  crystals  and  these 
forms  are  characteristic  as  in  the  case  of  plants  and  animals. 
An  elementary  knowledge  of  crystallography  is  essential  in 
the  study  of  minerals,  for  it  must  be  emphasized  that  chemical 
composition  alone  does  not  define  a  mineral.  Many  chemical 
elements  and  compounds  occur  in  two  or  more  distinct  forms 
which  are  known  as  polymorphs.  We  have,  for  example,  calcium 
carbonate  in  two  well-defined  forms:  calcite  and  aragonite. 
Although  these  have  exactly  the  same  chemical  composition, 
they  are  distinct  minerals,  for  they  each  have  a  distinctive  crystal 
form  and  distinctive  physical  properties  such  as  cleavage  and 
specific  gravity. 

Another  reason  for  studying  crystallography  is  the  ease  of 
identification  of  many  minerals  by  their  crystal  form  alone. 
Again,  a  knowledge  of  crystal  morphology  is  essential  to  a  proper 
understanding  of  physical  properties  because  there  is  an  intimate 
relation  between  the  crystal  form  and  physical  properties. 

The  physical  properties  of  minerals  are  important  because  the 
external  crystal  form  is  often  lacking.  The  external  form, 
however,  is  the  result  of  an  internal  structure  and  the  internal 
structure  is  reflected  in  the  physical  properties.  A  few  of  the  phys- 
ical properties,  such  as  the  specific  gravity,  are  independent  of  the 
direction  (scalar  properties),  but  most  of  them  depend  upon  the 
crystallographic  direction  and  so  are  called  vectorial.  Of  all  the 
vectorial  properties,  the  optical  properties  are  the  most  important 
in  the  description  and  determination  of  minerals.  The  polarizing 
microscope  may  be  used  to  determine  practically  all  of  the  optical 
properties.  This  method  has  a  great  advantage  over  chemical 
methods  in  that  a  very  small  quantity  of  material  suffices  and 


INTRODUCTION  3 

also  because  two  or  more  substances  may  be  recognized  in  the 
presence  of  each  other. 

The  chemical,  morphological,  and  physical  properties  are  of 
practically  equal  importance.  One  is  very  much  handicapped  if 
he  neglects  any  one  of  these  three  groups. 

After  the  properties  of  a  mineral  are  ascertained  and  the 
mineral  determined,  there  yet  remain  the  facts  of  its  occurrence 
and  association,  and  the  problem  of  its  origin.  The  role  that  the 
mineral  plays  in  nature  is  fully  as  important  and  interesting  as 
the  properties  of  the  mineral.  This  is  probably  the  most  fas- 
cinating branch  of  mineral  science.  It  includes  not  only  the 
study  of  rocks  but  also  the  study  of  mineral  deposits  in  general, 
among  which  ore-deposits  are  prominent.  The  occurrence  of 
certain  minerals  in  certain  rocks  and  mineral  deposits  is  charac- 
teristic, and  the  mineral  associations  are  more  or  less  typical. 


PART  I 

THE  PROPERTIES  OF  MINERALS 

THE  CHEMICAL  PROPERTIES  OF  MINERALS 

A.  CHEMICAL  PRINCIPLES 

As  minerals  are  naturally  occurring  substances  of  more  or 
less  definite  chemical  composition,  a  knowledge  of  inorganic 
chemistry  is  the  foundation  of  the  student's  work  in  mineralogy. 
While  it  is  true  that  a  few  physical  tests  often  serve  to  identify 
a  mineral,  a  more  or  less  complete  chemical  analysis  is  frequently 
necessary.  It  is  also  advisable  to  check  the  physical  tests  by  a 
chemical  examination.  Minerals  are  the  original  sources  of  the 
chemical  elements  and  compounds,  with  the  exception  of  carbon 
and  its  various  compounds  which  are  obtained  largely  from  plant 
and  animal  products.  Mineralogy  then  may  be  regarded  as  the 
natural  history  branch  of  inorganic  chemistry. 

1.  ELEMENTS 

With  the  possible  exception  of  some  inert  gases  of  the  atmos- 
phere (argon,  krypton,  neon,  and  xenon),  all  of  the  eighty-three 
known  elements  occur  in  minerals.  Some  are  exceedingly  rare 
and  confined  to  one  or  two  minerals,  while  others  are  very  common 
and  widely  distributed  in  minerals.  The  following  table  of  the 
elements  with  their  symbols  and  atomic  weights  shows  to  some 
extent  their  occurrence  in  minerals. 

Of  the  eighty-three  elements  enumerated,  only  about  twenty 
occur  as  minerals  in  the  free  or  uncombined  state.  They  are 
carbon,  sulfur,  selenium,  tellurium,  arsenic,  antimony,  bismuth, 
mercury,  copper,  silver,  gold,  lead,  iron,  nickel,  platinum,  palla- 
dium, iridium,  osmium,  tantalum,  and  tin.  This  leaves  out  of 

5 


INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Table  cf  Elements  with  their  Atomic  Weights 
^International  Committee,  1920-21) 


Element 

Symbol 

Atomic 
weight 

Occurrence 

Aluminum  

Al 
Sb 

27.1 
120.2 

Corundum,  AhOa 
Stibnite,  Sb2S3 

Argon   ".  

A 

39.9 

In  the  atmosphere 

Arsenic  

As 
Ba 

74.96 
137.37 

Arsenopyrite,  FeAsS 
Barite,  BaSO4 

Beryllium  (  =  Glucinum)  
Bismuth  
Boron         

Be 
Bi 
B 

9.1 
208.0 
10.9 

Beryl,  Be3Al2(SiO3)6 
Bismuthinite,  Bi2Sa 
Colemanite,  Ca2B6Oii.5H2O 

Br 

79.92 

Bromyrite,  AgBr 

Cadmium  

Cd 
Ca 

112.40 
40.07 

Greenockite,  CdS 
Calcite,  CaCOs 

Carbon             ...             

c 

12.005 

Graphite,  C 

Cerium           ....        

Ce 

140.25 

Monazite,  (Ce,La)PO4 

Cesium  
Chlorin   

Cs 
Cl 

132.81 
35.46 

Pollucite,  H2Cs2Al2(SiO3)5 
Halite,  NaCl 

Cr 

52  0 

Chromite,  (Fe,  Mg)(Cr,  A1)2O4 

Cobalt  

Co 
Cu 

58.97 
63  57 

Smaltite,  (Co,Ni)As2 
Chalcopyrite,  CuFeS2 

Dysprosium  
Erbium         

Dy 
Er 

162.5 
167.7 

With  the  rare  earths 
Sipylite,  ErNbO4 

Europium   

Eu 

152.0 

With  the  rare  earths 

Fluorin  

F 
Gd 

19.0 
157  3 

Fluorite,  CaF2 
In  gadolinite 

Ga 

70  1 

In  sphalerite 

Germanium       .... 

Ge 

72  5 

Argyrodite,  AgsGeSa 

Gold         

Au 

197.2 

Gold,  Au 

Helium       

He 

4.00 

In  uraninite 

Holmium  

Ho 
H 

163.5 
1  008 

In  gadolinite 
Water,  H2O 

In 

114  8 

In  sphalerite 

lodin         

I 

126.92 

lodyrite,  Agl 

Iridium  

Ir 
Fe 

193.1 

55  84 

Iridosmine,  (Ir.Os) 
Hematite,  Fe2Oa 

Kr 

82  92 

In  the  atmosphere 

Lanthanum  .... 

La 

139  0 

Lanthanite,  La2(CO3)3.9H2O 

Lead         

Pb 

207  20 

Galena,  PbS 

Lithium  

Li 
Lu 

6.94 
175  0 

Lepidolite,  K.Li.Al  silicate 
With  the  rare  earths 

Mg 

24  32 

Magnesite,  MgCOa 

Manganese 

Mn 

54  93 

Pyrolusite,  MnO2(H2O)4 

Mercury                       

Hg 

200.6 

Cinnabar,  HgS 

Molybdenum  
Neodymium  

Mo 

Nd 

96.0 
144.3 

Molybdenite,  MoS2 
In  monazite 

CHEMICAL  PROPERTIES  OF  MINERALS 
Table  of  Elements  with  their  Atomic  Weights — Continued 


Element 

Symbol 

Atomic 
weight 

Occurrence 

Neon  

Ne 

20.2 

In  the  atmosphere 

Nickel  
Niobium  (  =  Columbium)  
Niton  
Nitrogen  
Osmium  
Oxygen  
Palladium  
Phosphorus  

Ni    • 
Nb 
Nt 
N 
Os 
O 
Pd 
P 
Pt 

58.68 
93.1 
222.4 
14.008 
190.9 
16.00 
106.7 
31.04 
195  2 

Millerite,  NiS 
Columbite,  (Fe,Mn)(NbO3)2 
Radium  emanation 
Nitratine,  NaNOa 
Iridosmine,  (Ir.Os) 
Water,  H2O 
Palladium,  Pd 
Apatite,  CasFCPQOa 
Platinum,  Pt 

Potassium  
Praseodymium  
Radium  
Rhodium  
Rubidium  
Ruthenium  

K 
Pr 
R 
Rh 
Rb 
Ru 

39.10 
140.9 
226.0 
102.9 
85.45 
101.7 

Sylvite,  KC1 
In  monazite 
In  uraninite 
In  platinum 
In  rhodizite 
Laurite,  RuS2 

Sa 

150  4 

Sc 

45  0 

Se 

79  2 

Clausthalite  PbSe 

Si 

28  3 

Quartz    SiOz 

Silver  

Ag 

107  88 

Sodium  

Na 

23  00 

Halite    NaCl 

Strontium  
Sulfur  
Tantalum  

Sr 
S 
Ta 
Te 

87.63 
32.06 
181.5 
127  5 

Celestite,  SrSO4 
Sulfur,  S 
Tantalite,  Fe(TaO3)z 
Calaverite  AuTe2 

Tb 

159  2 

Thallium 

Tl 

204  0 

Lorandite   TIAsSa 

Thorium  
Thulium  
Tin  
Titanium  
Tungsten  
Uranium  

Th 
Tm 
Sn 
Ti 
W 

u 

232.15 
168.5 
118.7 
48.1 
184.0 
238  2 

Thorite,  ThSiO* 
In  gadolinite 
Cassiterite,  SnO2 
Rutile,  TiO2 
Wolframite,  (Fe,Mn)WO4 
Uraninite   UaOs 

Vanadium  

V 
Xe 

51.0 
130  2 

Vanadinite,  Pb6Cl(VOOa 

Ytterbium  

Yb 

173  5 

n      e  a  mosp 

Yttrium  
Zinc  
Zirconium  

Y 

Zn 

Zr 

89.33 
65.37 
90.6 

Xenotime,  YPOi 
Sphalerite,  ZnS 
Zircon,  ZrSiO4 

consideration  the  free  gases  of  the  atmosphere.  From  a  chemical 
standpoint  the  elements  may  be  divided  into  two  classes:  the 
metals  and  the  non-metals.  The  metals  include  such  elements  as 


8  INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

copper,  silver,  gold,  lead,  iron,  and  platinum.  Some  of  these 
occur  as  alloys,  such  as  electrum  (Au,Ag),  amalgam  (Ag,Hg), 
nickel-iron  (Fe,Ni),  and  iridosmine  (Ir,Os).  The  non-metals 
include  such  elements  as  oxygen,  hydrogen,  nitrogen,  phosphorus, 
and  sulfur.  Arsenic,  antimony,  and  bismuth  are  intermediate 
in  their  properties  between  metals  and  non-metals,  and  hence  are 
usually  called  semi-metals  or  metalloids. 

2.  CHEMICAL  COMPOUNDS 

Most  minerals  are,  of  course,  chemical  compounds,  or  combi- 
nations of  two  or  more  elements.  These  compounds  are  the 
chemical  types  recognized  by  chemists,  namely:  oxids,  acids, 
bases,  and  salts,  with  their  various  subdivisions. 

Acids  are  compounds,  the  dilute  water  solutions  of  which 
contain  hydrogen  ions.  According  to  the  theory  of  ions,  it  is  the 
hydrogen  ions  that  give  the  acid  properties  such  as  sour  taste 
and  the  change  of  blue  litmus  to  red.  The  strength  of  the  acid 
depends  upon  the  proportion  of  hydrogen  ions  present  or  upon 
the  degree  of  dissociation.  Hydrochloric  and  sulfuric  acids 
are  strong  acids,  while  carbonic  and  silicic  acids  are  weak  acids. 
Acids  are  compounds  of  hydrogen  with  the  halogens  (Cl,  Br,  I,  or 
F),  with  sulfur,  or  with  certain  radicals,  such  as  CO3,  864,  PO4, 
AsC>4,  AsS3,  AsS4,  SbS3,  SbS4.  These  are  called  acid  radicals. 
The  most  common  acids  are  those  containing  oxygen  and  hence 
are  known  as  oxygen  acids  or  oxy-acids.  The  oxy-acids  are  mono- 
basic (HN03),  dibasic  (H2SC>4),  tribasic  (H3PO4),  or  tetrabasic 
(H4SiO4),  according  as  they  have  one,  two,  three,  or  four  replace- 
able H  atoms.  The  polybasic  acids,  as  they  are  called,  are 
capable  of  forming  condensed  acids  by  subtracting  water.  This 
is  especially  prominent  with  the  silicic  acids.  Orthosilicic  acid  is 
H4SiO4;  H4SiO4-H2O  =  H2SiO3,  metasilicic  acid;  2H4SiO4- 
H2O  =  H6Si2O7,  diorthosilicic  acid;  2H4SiO4  -  3H2O  =  H2Si2- 
Os,  dimetasilicic  acid.  There  are  also  H4Si3O8  (3H4SiO4  - 
4H2O),  H8Si3Oio  (3H4SiO4  -  2H2O),  and  still  other  possible 
silicic  acids. 


CHEMICAL  PROPERTIES  OF  MINERALS  9 

The  replacement  in  the  oxy-acids  of  0  by  S  gives  compounds 
called  sulfo-acids.  Thus  H3AsO4  is  arsenic  acid  or  oxy-arsenic 
acid,  while  H3AsS4  is  sulf arsenic  acid.  H3AsO3  is  arsenious  acid, 
while  H3AsS3  is  sulf  arsenious  acid.  Various  condensed  acids, 
which  are  entirely  analogous  to  the  condensed  oxy-acids,  are  de- 
rived from  the  above  by  the  subtraction  of  H2S.  Thus  we  have 
HAsS2(H3AsS3  —  H2S),  metasulf arsenious  acid,  and  H4As2S7 
(2H3AsS3  —  H2S),  pyrosulf arsenious  acid.  Very  few  of  these 
acids  exist  either  as  minerals  or  prepared  compounds,  but  salts 
of  all  of  them  are  known  as  minerals. 

Bases  are  compounds  the  dilute  water  solutions  of  which  con- 
tain hydroxyl  (OH)  ions.  The  hydroxyl  ions  give  the  basic 
properties  such  as  soapy  feel,  and  the  change  of  red  litmus 
to  blue.  The  strength  of  the  base  is  proportional  to  the  number 
of  hydroxyl  ions  present.  The  strong  bases  such  as  KOH  and 
NaOH  are  called  alkalies.  Weak  bases  are  represented  by 
Fe(OH)3  and  A1(OH)3.  Among  the  bases  represented  by  min- 
erals are  Mg(OH)2,  Mn(OH)2,  Al(OH),,  HA1O2[A1(OH)3-H2O], 
and  HFeO2[Fe(OH)3  -  H2O]. 

Oxids  are  compounds  of  the  elements  with  oxygen.  Elements 
the  oxids  of  which  form  bases  with  water  are  called  metals. 
These  oxids  are  called  basic  anhydrids  for  this  reason.  Elements 
the  oxids  of  which  form  acids  with  water  are  called  non-metals. 
These  oxids  are  called  acid  anhydrids. 

Salts  are  compounds  formed  by  the  union  of  bases  with  acids; 
the  metal  of  the  base  unites  with  the  non-metal  or  acid  radical 
of  the  acid  to  form  the  salt,  while  the  hydroxyl  of  the  base  unites 
with  the  hydrogen  of  the  acid  to  form  water  thus:  NaOH  +  HC1 
=  NaCl  +  H2O.  In  dilute  solutions  salts  are  dissociated  into  two 
parts  or  ions  as  they  are  called.  The  metal  forms  one  ion,  called 
the  cation,  while  the  non-metal  or  acid  radical  forms  the  other 
ion,  called  the  anion. 

Among  salts  we  may  distinguish  halogen  salts,  oxy-salts,  and 
sulfo-salts  corresponding  to  the  acids  of  which  they  are  the 
derivatives.  The  following  represent  salts  found  as  minerals: 


10 


INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Sulfid,  PbS  (galena). 
Selenid,  PbSe  (clausthalite). 
Tellurid,  PbTe  (altaite). 
Arsenid,  FeAs2  (lollingite). 
Antimonid,  Ag3Sb  (dyscrasite). 
Sulfarsenite  Ag3AsS3  (proustite). 
Sulfarsenate  Cu3AsS4  (enargite). 
Sulfantimonite    Ag3SbSa    (pyrargy- 

rite). 
Sulfantimonate,    CusSbS4     (famati- 

nite). 

Sulfoferrite,  CuFeS2  (chalcopyrite). 
Sulfochromite,     FeCr2S4    (daubree- 

lite). 

Sulfovanadate,  Cu3VS4  (sulvanite). 
Sulfogermanate,    AgsGeSe    (argyro- 

dite). 

Sulfostannite,  PbSnS2  (teallite). 
Chlorid,  AgCl  (cerargyrite). 
Bromid,  AgBr2  (bromyrite). 
lodid,  Agl  (iodyrite). 
Fluorid,  CaF2  (fluorite). 
Fluosilicate,  K2SiF6  (hieratite). 
Carbonate,  CaCO3  (calcite). 
Meta-aluminate,  MgAl2O4  (spinel). 
Metaferrite,  FeFe2O4  (magnetite). 


Metachromite  FeCr2O4  (chromite). 
Metaniobate,     Fe(NbO3)2    (colum- 

bite). 
Metatantalate,    Fe(TaO3)2    (tanta- 

lite). 

Phosphate,  LiFePO4  (triphylite). 
Arsenate,  FeAsO4-H2O  (scorodite). 
Vanadate,  BiVO4  (pucherite). 
Antimonate,  Ca2Sb2O7  (atopite). 
Nitrate,  NaNO3  (nitratine). 
Borate,  A1BO3  (jeremejevite). 
Sulfate,  BaSO4  (barite). 
Chromate,  PbCrO4  (crocoite). 
Selenite,  CuSeO3-2H2O  (chalcomen- 

ite). 
Tellurite,      Fe(TeO3)34H2O      (dur- 

denite). 

Tungstate,  CaWO4  (scheelite). 
Molybdate,  PbMoO4  (wulfe.nite). 
Metatitanate,  CaTiO3  (perovskite). 
Orthosilicate  ( Mg,  Fe )  2SiO 4  (olivine ) . 
Metasilicate,  CaSiOs  (wollastonite). 
Trisilicate,  KAlSi3O8  (orthoclase) . 
Dimetasilicate,   LiAl(Si2O5)2    (peta- 

lite). 
Diorthosilicate,Pb3Si2O7(barysilite). 


All  the  above  are  normal  salts ;  that  is,  all  the  hydrogen  of  the 
acid  or  hydroxyl  of  the  base  has  been  replaced  by  metals  or  by 
acid  radicals  respectively.  A  compound  in  which  only  part  of 
the  hydrogen  of  the  acid  has  been  replaced  by  a  metal  is  called 
an  acid  salt.  Among  minerals  we  have  KHSCU,  and  H2CuSiO4, 
which  are  called  acid  potassium  sulfate,  and  acid  copper  silicate 
respectively.  A  compound  in  which  only  part  of  the  hydroxyl 
of  the  base  is  replaced  by  an  acid  radical  is  called  a  basic  salt. 
Among  minerals  we  have  Cu2(OH)2CO3,  Cu4(OH)6Cl2,  Cu2(OH)- 
AsO4,  and  many  others.  These  three  compounds  are  called  basic 
copper  carbonate,  chlorid,  and  arsenate  respectively. 

The  formulae  of  some  minerals  are  written  as  though  they  consist 
of  two  or  more  separate  molecules.  These  are  called  molecular 


CHEMICAL  PROPERTIES  OF  MINERALS  11 

compounds  for  want  of  a  better  name.  Among  molecular  com- 
pounds are  double  salts  and  hydrates  or  hydrous  salts.  Double 
salts  are  (1)  salts  composed  of  two  metals  with  a  common  acid 
radical  (example,  dolomite  CaCO3-MgCO3),  (2)  salts  of  a  single 
metal  with  two  distinct  acid  radicals  (example,  arsenopyrite 
FeS2-FeAs2),  or  (3)  salts  in-  which  both  the  metal  and  acid 
radical  are  different  (example,  kainite  MgSO4'KCl*3H2O). 

Acid  and  basic  salts  may  also  be  written  in  the  form  of  double 
salts.  KHSO4  =  K2SO4-H2SO4,  Cu2(OH)2CO3  =  CuCO3'Cu- 
(OH)2.  Another  kind  of  compound  is  Sb2S2O,  antimony 
oxy-sulfid,  which  may  be  written  2Sb2S3-Sb2O3(3Sb2S2O). 
Similarly  Pb2OCl2  (or  PbCl2'PbO)  is  lead  oxy-chlorid  and 
Pb2Cl2CO3  (or  PbCl2'PbCO3)  is  lead  chloro-carbonate. 

Acid  and  basic  salts  when  heated  in  the  closed  tube  at  a 
relatively  high  temperature  (usually  above  200°  C.)  give  off 
water  and  this  water  is  called  water  of  constitution. 

In  other  compounds  water  is  more  loosely  held,  and  when  heated 
is  driven  off  at  a  temperature  varying  from  about  100°  C.  to  200°  C. 
This  is  the  so-called  water  of  crystallization,  but  this  term  is  a  mis- 
nomer, as  water  is  not  necessary  for  crystallization.  Most  an- 
hydrous minerals  occur  in  well  formed  crystals.  Salts  which  give 
off  a  definite  amount  of  water  at  low  temperatures  are  called  hy- 
drates or  hydrous  salts.  The  formula  is  written  as  if  they  contain 
water  as  such.  Examples:  hydrous  calcium  sulfate,  Ca,SO4'2H2O 
(gypsum);  hydrous  sodium  borate,  Na2B407'10H2O  (borax). 
There  may  be  various  hydrates,  for  example,  MgSO4*H2O  (kieser- 
ite);  MgSO4-6H2O  (hexahydrite) ,  and  MgSO47H2O  (epsomite). 

The  following  are  examples  of  complicated  salts  which  occur 
as  minerals:  HNa3(C03)2'2H20,  hydrous  acid  sodium  car- 
bonate (trona);  Fe4(OH)2(S04)5'17H2O,  hydrous  basic  ferric 
sulfate  (copiapite). 

3.  DERIVATION  OF  CHEMICAL  FORMULA 

Most  minerals  have  a  fairly  definite  chemical  composition  and 
hence  may  be  represented  by  a  formula.  The  formula  is  obtained 


12          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

by  dividing  the  percentage  composition  of  the  various  elements  or 
radicals  by  the  corresponding  combining  weights  as  found  in  a 
table  of  atomic  weights  (pages  6-7).  The  ratio  of  these 
expressed  in  the  simplest  whole  numbers  possible  gives  the 
empirical  formula.  Example:  An  analysis  of  chalcopyrite  from 
Phoenix ville,  Pennsylvania,  furnished  J.  Lawrence  Smith  the 
results  of  column  I. 

I  II  III  IV 

Cu 32.85-^63.6  =0.516  34.57 

Fe 29.93-5-55.8  =0.536  30.54 

S 36.10-7-32.1  =1.121  34.89 

Pb 0.35 

Dividing  by  the  combining  weights  given  in  the  second  column 
we  have  the  figures  of  the  third  column,  lead  being  omitted. 
These  numbers  are  nearly  in  the  ratio  1:1:  2,  hence  the  formula 
CuFeS2.  The  theoretical  percentages  for  chalcopyrite  are 
given  in  the  fourth  column. 

Discrepancies  in  analyses  may  be  explained  in  a  number  of 
ways.  It  is  very  common  for  similar  metals  or  acid  radicals 
to  replace  each  other  in  varying  proportions.  In  this  case  the 
combining  ratios  of  replacing  metals  or  acid  radicals  are  added 
together.  An  analysis  of  brown  sphalerite  from  Roxbury, 
Connecticut,  gave  Caldwell  the  percentage  composition  of 
column  I.  Dividing  by  the  atomic  weights  of  column  II  we 
have  the  combining  ratios  of  III.  The  sum  of  the  combining 
ratios  for  Zn  and  Fe  (1.033)  is  to  the  combining  ratio  for  S 
(1.039)  practically  as  1:1.  Hence  the  formula  is  (Zn,Fe)S, 
which  means  that  iron  replaces  zinc  in  varying  amounts.  Anal- 
yses of  sphalerite  show  an  iron  content  varying  from  nil  up  to 
18  per  cent. 

I  II  III  IV 

Zn ^3.36  -J-65.4  =0.969  \ 

Fe 3.60  -7-55.8  =0.063  J 

S 33.36  -4-32.1  =  1.039      1.039 

Analyses  of  oxids,  haloids,  sulfids,  and  sulfo-salts  are  given  as 


CHEMICAL  PROPERTIES  OF  MINERALS  13 

percentages  of  the  elements.  This  cannot  be  done  with  the 
oxygen  salts  as  there  is  no  way  of  determining  oxygen  directly; 
therefore  the  percentage  composition  of  the  oxygen  salts  must  be 
expressed  either  as  oxids,  or  as  metals  and  acid  radicals.  At 
present  it  is  customary  to  use  the  oxids.  This  is  in  accordance 
with  the  electro-chemical  theory  of  Berzelius  in  which  dualistic 
formulae  were  used.  Thus  FeSCX  was  considered  as  FeOSO3, 
FeO  being  the  base  or  electropositive  radical  and  SO3,  the  acid  or 
electronegative  radical.  Although  these  views  are  considered 
antiquated  by  modern  chemists,  still  the  custom  is  to  employ 
the  basic  and  acid  anhydrids  in  stating  the  results  of  analysis. 
Thus  CaSC>4  is  given  as  CaO  and  SO3.  The  method  of  giving 
the  metals  and  acid  radical  is  preferable  if  haloids  or  sulfids  are 
present.  In  the  ordinary  method  there  is  an  excess  of  oxygen 
equivalent  to  the  amount  of  halogen  or  sulfur  present  which 
must  be  deducted.  For  example,  apatite  is  Ca5F(PO4)3.  The 
calculated  percentage  compositions  are:  CaO  =  55.5,  P2O5  = 
42.3,  F  =  3.8;  total  =  101.6.  The  excess  over  100  per  cent  is 
due  to  the  fact  that  only  part  of  the  calcium  is  combined  with  the 
oxygen,  as  can  be  seen  by  expressing  the  formula  in  another  way : 
9CaO3P2(VCaF2.  The  oxygen  equivalent  of  F  is  >^O  with 
atomic  weight  of  8.  The  percentage  compositions  given  above 
have  been  figured  on  the  basis  of  a  formula  weight  of  512.5 
(504.5+8).  [512.5  :  504.5  :  :  101.6  :  100].  A  much  better  way  is 
to  express  the  percentage  compositions  thus :  Ca  =  39.7,  F  =  3.8, 
P04  =  56.5;  total  =  100.0. 

In  the  case  of  hydrous,  acid,  or  basic  salts  of  any  kind,  the 
water  percentage  is  given,  as  the  determination  of  water  is  often 
a  practical  means  of  identifying  a  mineral. 

4.  VARIATIONS  IN  THE  CHEMICAL  COMPOSITION  OF  MINERALS 

In  the  foregoing  discussion  it  was  assumed  that  minerals 
have  a  definite  chemical  composition.  Strictly  speaking,  this  is 
true  of  only  a  comparatively  few  minerals  such  as  some  specimens 


14          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

of  quartz,  calcite,  and  a  few  others.  While  other  minerals 
approach  definiteness  of  chemical  composition  many  others  are 
far  from  being  definite. 

In  the  first  place,  it  should  be  emphasized  that  much  of  the 
variation  from  the  theoretical  values  of  chemical  formulae  is  due 
to  mechanical  impurities.  Many  apparently  homogeneous 
substances  prove  on  microscopic  examination  to  be  mixtures  of 
two  or  more  substances.  For  example,  wollastonite  (CaSiO3) 
usually  seems  to  effervesce  in  acids  but  the  effervescence  is 
due  to  admixed  calcite.  So-called  cupriferous  pyrite  contains 
small  amounts  of  chalcopyrite,  as  may  be  proved  by  examining 
polished  surfaces  of  specimens  with  the  reflecting  or  metallo- 
graphic  microscope.  But  complexity  in  chemical  composition 
is  not  always  proof  of  mechanical  mixtures. 

In  homogeneous  minerals  the  departure  from  fixed  chemical 
composition  can  be  explained  in  two  or  three  different  ways: 

(1)  by  solid  solution  of  which  isomorphism  is  a  special  case,  and 

(2)  by  the  fact  that  the  mineral  is  of  colloidal  origin. 

6.  ISOMORPHISM 

Many  compounds  of  similar  chemical  composition,  especially 
salts  with  the  same  acid  radicals  and  related  metals,  have  almost 
identical  crystal  forms.  Such  compounds  are  said  to  be  isomor- 
phous.  Isomorphous  substances  have  similar  form,  but  except 
in  the  isometric  system  this  does  not  mean  that  the  form  is  iden- 
tical. For  example,  the  angle  (110  :  110)  for  barite,  BaSO4,  is 
78°  22^',  while  for  celestite,  SrSO4,  the  corresponding  angle  is 
75°  50',  and  for  anglesite,  PbSO4,  it  is  76°  16^'.  Barite,  celestite, 
and  anglesite  thus  form  an  isomorphous  group.  Among  promi- 
nent isomorphous  groups  of  minerals  are  the  following: 

Pyrite  FeS2 

Smaltite  (Co,Ni)As2 

Cobaltite  CoAsS 

Gersdorffite  NiAsS 


CHEMICAL  PROPERTIES  OF  MINERALS 


15 


Ruby  Silvers 


Marcasite 

FeS2 

Arsenopyrite 

FeAsS 

Lollingite 

FeAs2 

Glaucodot 

(Co,Fe)AsS 

Safflorite 

CoAs2 

Rammelsbergite 

NiAs2 

Tetrahedrite 

Cu3SbS3  +  z(Fe,Zn)6Sb2S9 

Tennantite 

Cu3AsS3  +  z(Fe,Zn)6As2S9 

Proustite 

Ag3AsS3 

Pyrargyrite 

Ag3SbS3 

Corundum 

A1203 

Hematite 

Fe203 

Cassiterite 

SnO2 

Rutile 

TiO2 

Diaspore 

A1(OH)3-A12O3 

Goethite 

Fe(OH)3-Fe2O3 

Manganite 

Mn(OH)3-Mn2O3 

Calcite 

CaCO3 

Magnesite 

MgCO3 

Siderite 

FeCO3 

Rhochrosite 

MnCO3 

Smithsonite 

ZnCO3 

Aragonite 

CaCO3 

Strontianite 

SrCO3 

Witherite 

BaCO3 

Cerussite 

PbCO3 

Fluor-apatite 

CaioF2(P04)6 

Chlor-apatite 

Ca10Cl2(P04)6 

Dahllite 

Ca10(C03)(P04)6 

Voelckerite 

CaioO(PO4)6 

Svabite 

Cai0F2(AsO4)6 

Pyromorphite 

PbioCl2(P04)6 

Mimetite 

PbioCl2(AsO4)6 

Vanadinite 

Pb10Cl2(V04)6 

Barite 

BaSO4 

Celestite 

SrSO4 

Anglesite 

PbS04 

Alunite 

KA13(OH)6S04 

Jarosite 

KFe3(OH)6SO4 

16          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Ilmenite 

FeTiO3 

Geikielite 

MgTiOg 

Pyrophanite 

MnTiO3 

Senaite 

(Fe,Mn,Pb)TiO3 

Grossularite 

Ca«Al2(SiO4)« 

Pyrope 

Mg3Al2(SiO4)3 

Almandite 

Fe3Al2(Si04)3 

Spessartite 

Mn3Al2(SiO4)3 

Andradite 

Ca3Fe2(SiO4)3 

Uvarovite 

Ca3Cr2(Si04)3 

Garnet 


Many  isomorphous  compounds  are  capable  of  crystallizing 
together  in  various  proportions  and  thus  form  what  are  known  as 
isomorphous  mixtures.  There  are  many  such  cases  among  min- 
erals which  fact  is  very  useful  in  interpreting  mineral  analyses. 
Chromite  is  an  isomorphous  mixture  of  FeCr2O4,  FeFe2O4, 
MgCr204,  MgAl204,  and  FeAl2O4,  all  of  which  are  known  as 
minerals.  The  formula  used  to  express  the  chemical  composition 
is  as  follows:  (Fe",  Mg)  (Cr,  Al,  Fe///)204,  which  means  that  the 
combined  proportion  of  chromium,  aluminum,  and  ferric  iron  is 
twice  (molecularly)  that  of  ferrous  iron  and  magnesium  together. 
Among  prominent  isomorphous  mixtures  are:  sphalerite  (Zn,Fe)S, 
smaltite  (Co,Ni)As2,  columbite  (Fe,Mn)(Nb,Ta)2O6,  campylite 
Pb5Cl(As,P)3Oi2,  endlichite  Pb5Cl(As,V)3Oi2,  pisanite  (Fe,Cu)- 
SO47H2O,  wolframite  (Fe,Mn)WO4,  actinolite  Ca(Mg,Fe)3- 
(SiO,)4,  and  epidote  Ca2(Al,Fe)3(OH)(SiO4)3.  The  garnets  are 
isomorphous  mixtures  of  the  compounds  mentioned  above. 
It  is  rare  to  find  an  analysis  of  garnet  that  will  correspond 
exactly  to  any  one  of  these  formulae. 

The  physical  properties  of  isomorphous  mixtures  vary  con- 
tinuously; for  this  reason  the  term  solid  solution  is  sometimes 
used.  The  best  test  of  isomorphism  is  the  ability  to  form  mixed 
crystals. 

6.    SOLID  SOLUTIONS  NOT  DUE  TO  ISOMORPHOUS  MIXTURES 

While  most  of  the  variations  in  the  chemical  composition  of 
crystalline  minerals  may  be  explained  by  isomorphism  there  are 


CHEMICAL  PROPERTIES  OF  MINERALS  17 

some  cases  which  cannot  be  so  explained.  For  example,  pyrrho- 
tite  contains  a  slight  excess  of  sulfur  over  that  required  for  the 
formula  FeS.  Formerly  this  was  expressed  by  the  formula 
FenSn+i,  in  which  n  varies  from  5  to  15,  in  accordance  with 
the  law  of  multiple  proportions  and  the  belief  that  every  mineral 
has  a  definite  composition.  This  we  know  to  be  false  and  the 
modern  way  of  expressing  the  chemical  composition  of  pyrrhotite 
is  by  means  of  the  following  formula:  FeS(S)x,  which  signifies 
that  pyrrhotite  is  regarded  as  a  solid  solution  of  sulfur  in  ferrous 
sulfid. 

Another  example  of  solid  solution  is  nepheline,  which  contains 
an  excess  of  silica  over  that  required  for  the  formula  (Na,K)- 
AlSi04. 

The  color  of  many  minerals  is  due  to  a  solid  solution  of  some 
pigment  (either  organic  or  inorganic)  in  the  mineral. 

7.  MINERALS  OF  COLLOIDAL  ORIGIN 

Most  of  the  amorphous  minerals,  such  as  opal,  cliachite,  and 
collophane,  are  of  colloidal  origin.  They  are  apparently  due  to 
the  gradual  hardening  or  setting  of  a  gelatinous  mass.  Such 
hardened  gels  were  called  porodine  by  Breithaupt.  The  mech- 
anism of  the  hardening  of  a  gel  is  not  well  understood  nor  is  the 
nature  of  the  gel  itself,  but  some  of  the  hardened  gels  are  common 
minerals  and  so  deserve  our  attention. 

When  a  substance  is  precipitated  in  the  form  of  minute  particles 
intermediate  in  size  between  ordinary  visible  suspensions  and 
solutions,  it  possesses  peculiar  properties  due  to  the  enormous 
surface  exposed.  The  phenomenon  of  surface  tension  plays  an 
important  part  in  determining  these  properties.  Such  a  state 
or  condition  of  a  substance  is  called  the  colloidal  or  dispersed 
state.  Colloidal  substances,  or  more  accurately  speaking  sub- 
stances in  the  colloidal  condition,  on  account  of  the  great  sur- 
faces exposed,  have  a  tendency  to  take  up  or  adsorb  other  sub- 
stances from  the  solutions  in  which  they  are  formed.  This 
adsorption  seems  to  be,  to  a  large  extent,  selective,  and  for  this 


18          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

reason  most  of  the  amorphous  minerals  of  colloidal  origin  ap- 
proach in  chemical  composition  that  of  the  corresponding  crys- 
talline mineral.  This  was  first  emphasized  by  the  Austrian 
mineralogist,  Cornu,  in  1909.  Cellophane,  for  example,  is  very 
much  like  crystalline  dahllite,  SCasCPO^'CaCOs,  in  chemical 
composition.  Nearly  all  the  well-established  amorphous  minerals 
have  crystalline  equivalents.  Opal  is  the  amorphous  equivalent 
of  chalcedony  and  quartz.  Cliachite  is  the  amorphous  equivalent 
of  gibbsite  [Al(OH)  J.  Turyite  is  probably  the  amorphous  equiva- 
lent of  hematite.  Psilomelane  is  the  amorphous  equivalent  of 
hollandite  or  romanche"ite.  Pitchblende  is  probably  the  amor- 
phous equivalent  of  uraninite.  Halloysite  is  the  amorphous 
equivalent  of  kaolinite.  Cornuite,  recently  described  by  the 
author,  is  the  amorphous  equivalent  of  chrysocolla.  Some 
common  minerals  such  as  calcite,  barite,  fluorite,  etc.,  have 
no  amorphous  equivalents,  or  at  least  they  have  not  yet  been 
described. 

Because  of  adsorption  the  amorphous  minerals  vary  some- 
what in  chemical  composition  but  there  is  also  variation  in  the 
water  content.  Practically  all  the  amorphous  minerals  contain 
water,  the  reason  being  that  they  are  hardened  hydrogels,  that  is, 
gels  formed  in  water  solution.  While  the  water  is  adsorbed  when 
the  colloid  is  first  formed,  there  is  a  probability  of  solid  solution 
being  formed  by  the  diffusion  of  the  water  and  also  of  the  other 
adsorbed  substances.  The  water,  however,  is  variable  in  amount 
and  so  may  be  represented  by:  (H2O)X.  Although  practically 
always  present,  the  water  is  probably  not  essential. 

8.  POLYMORPHISM 

Something  besides  chemical  composition  must  be  taken  in 
account  in  the  study  of  minerals,  for  it  is  a  well-known  fact  that 
many  chemical  substances  exist  in  two  or  more  distinct  forms. 
That  is,  they  occur  in  crystals  with  different  internal  arrange- 
ments of  atoms,  usually  belong  to  different  crystal  systems,  and 
have  dissimilar  physical  properties. 


CHEMICAL  PROPERTIES  OF  MINERALS 


19 


Such  compounds  are  called  polymorphs.  A  familiar  example 
is  carbon  which  occurs  as  graphite  in  soft,  opaque,  hexagonal 
crystals,  and  as  diamond  in  very  hard,  transparent,  isometric 
crystals.  Polymorphous  elements  like  carbon  sare  called  allo- 
tropic.  Among  polymorphous  minerals  may  be  mentioned  the 
following:  V 


FeS2 


/  Diamond — Isometric 
\  Graphite — Hexagonal 

(Py  r  ite — Isom  etric 
Marcasite — Orthorhombic 


CaCO, 


Calcite — Hexagonal 
Aragonite — Orthorhombic 

(Orthoclase — Monoclinic 
Adularia — Monoclinic 
Microcline — Triclinic 


Si02 


a-Quartz — Hexagonal  (As.3A2) 

/3-Quartz — Hexagonal  (A6.6A2) 

Chalcedony— (?) 

Tridymite — Hexagonal  (Symmetry  unknown) 

Cristobalite — Isometric 


f  Rutile — Tetragonal  (6  =  0.64) 
TiO2 1  Octahedrite— Tetragonal  (6  =  1.77) 
Brookite — Orthorhombic 


Ca2Al3(OH)(SiO4); 


Al2SiO5 


H2Mg3Si2O9 


Zoisite — Orthorhombic 
Clinozoisite — Monoclinic 

Kyanite — Triclinic 

Andalusite— Orthorhombic  (d  =  0.986,  <i  =  0.702) 

Sillimanite — Orthorhombic  (&  =0.970,  6  =  ? 

Antigorite — Orthorhombic 
Chrysotile — Orthorhombic 


Polymorphism  seems  to  be  a  general  phenomenon  of  nature. 

Numerous  examples  of  polymorphism  occur  in  prepared  com- 
pounds. Sulfur  may  be  prepared  in  at  least  four  modifications: 
a-sulfur  (orthorhombic);  0-sulfur,  (monoclinic) ;  7-sulfur,  (also 
monoclinic,  but  with  different  axial  ratio  from  /3-sulf ur) ;  5-sulfur, 


20          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

(rhombohedral) .  Mercuric  iodid,  HgI2,  exists  in  a  red  tetragonal 
modification  (from  solutions),  and  also  in  a  yellow  orthorhombic 
modification  (from  fusion  or  sublimation). 

In  most  cases  it  is  the  temperature  which  determines  the 
modification  formed.  Thus  calcite  forms  from  aqueous  solutions 
below  30°  C.,  while  aragonite  forms  above  30°  C.  For  example, 
the  crust  of  CaCO3  often  formed  in  a  tea-kettle  is  aragonite. 

The  silica  minerals  furnish  the  best  known  example  of  poly- 
morphism. A  discussion  of  the  stability  relations  of  these 
minerals  will  be  found  on  page  265. 

B.  BLOWPIPE  ANALYSIS 

The  advantage  of  blowpipe  analysis  lies  in  the  fact  that  the 
tests  are  simple,  the  apparatus  portable,  and  the  reagents  few 
in  number.  By  means  of  the  blowpipe  an  intense  heat  (about 
1500°  C.)  can  be  obtained  on  a  small  scale,  and  a  variety  of  chem- 
ical effects  can  be  brought  about.  At  the  same  time  it  is  the 
author's  opinion  that,  with  a  few  exceptions,  blowpipe  analysis  is 
of  value  only  in  the  determination  of  minerals. 

Blowpipe  analysis  will  be  discussed  under  four  headings:  (1) 
apparatus,  (2)  reagents,  (3)  operations,  and  (4)  select  tests. 
Section  3  may  be  used  in  preliminary  tests  and  also  as  determi- 
native tables,  while  tests  for  the  metals  and  acid  radicals  may  be 
found  in  section  4  arranged  alphabetically  by  elements. 

1.  APPARATUS  USED  IN  BLOWPIPE  ANALYSIS 

Blowpipe.  The  blowpipe  is  made  in  a  variety  of  forms.  The 
simplest  blowpipe  is  a  brass  tube  about  10  inches  long,  bent  at 
one  end.  A  bulb  is  sometimes  added  to  condense  moisture.  A 
more  elaborate  form  is  a  nickel-plated  tube  with  a  moisture 
chamber  at  one  end  and  a  smaller  tube  at  right  angles  which  is 
provided  with  either  a  brass  or  a  platinum  tip. 

Where  gas  is  available,  the  gas  blowpipe  is  undoubtedly  the 
most  convenient  form  on  account  of  the  perfect  control  of  the 


CHEMICAL  PROPERTIES  OF  MINERALS  21 

flame.  The  gas  blowpipe  is  similar  to  the  nickel-plated  form 
just  described,  but  the  smaller  right-angled  tube  is  a  double 
one;  the  inner  one  for  air,  the  outer  one  for  gas. 

Fuel.  Gas  is  the  most  convenient  and  commonly  used  fuel. 
If  a  Bunsen  burner  is  used,  it  is  well  to  use  a  small  tube  which  fits 
the  top  of  the  Bunsen  burner. and  is  provided  with  a  flange  in 
which  the  tip  of  the  blowpipe  rests.  The  luminous  flame  of  the 
Bunsen  burner  should  be  used  with  the  blowpipe. 

Where  gas  is  not  available,  alcohol,  lard  oil,  or  olive  oil  may  be 
burned  in  a  lamp  provided  with  a  wick.  Candles  are  even  more 
convenient.  For  field  use  a  good  combination  is  alcohol  for 
heating,  and  candles  for  use  with  the  blowpipe. 

Charcoal.  Slabs  of  charcoal  about  4  inches  long,  1  inch  wide, 
and  %  inch  thick,  are  used  as  a  blowpipe  support.  They  may  be 
purchased  from  dealers  in  chemical  apparatus. 

Plaster.  A  paste  of  plaster-of-Paris  with  water  is  poured  out 
on  oiled  glass  in  sheets  about  J4  inch  thick.  Before  hardening, 
it  is  marked  off  in  rectangles  about  4  inches  long  and  1  inch  wide. 

Platinum  Tipped  Forceps.  These  forceps  are  essential  for 
testing  the  fusibility  of  minerals,  and  are  useful  for  other  pur- 
poses. Arsenic,  antimony,  lead,  and  copper  minerals  should  be 
fused  on  charcoal,  for  these  metals  alloy  with  platinum. 

Hammer.  A  small  square-faced  hammer  of  about  one-fourth 
pound  weight  is  indispensable. 

Anvil.  A  small  block  of  steel,  square  or  rectangular  in  cross- 
section,  and  about  J£  inch  thick,  is  convenient  for  powdering 
minerals  and  has  many  other  uses. 

Platinum  Wire.  No.  27  platinum  wire  is  the  best  size  for 
general  use.  The  wire  may  be  fused  into  a  piece  of  glass  tubing, 
or  held  in  a  special  holder  made  for  the  purpose. 

Test  Tubes.  The  most  convenient  size  is  4  inches  long  and 
J/£  inch  in  diameter. 

Glass  Tubing.  Soft  glass  tubing  of  7  mm.  outside  diameter  is 
best  for  most  purposes,  but  it  is  well  to  have  a  variety  of  sizes. 
For  some  tests  hard  glass  tubing  is  preferable. 


22          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Watch  Glasses.  These  are  needed  especially  for  solubility 
tests.  The  best  size  is  2  inches  in  diameter. 

Magnet.  A  magnetized  knife-blade  answers  the  same  purpose 
and  is  more  convenient. 

Lens.  A  Coddington  or  aplanatic  triplet  of  J4-inch  focus  is 
recommended.  This  gives  a  magnification  of  about  15  diameters. 

A  triangular  file,  blue  glass  (or  Merwin's  flame  color  screen), 
funnels,  and  filter-paper  are  also  essential. 

The  following  pieces  of  apparatus  are  not  essential,  but  will  be 
found  very  useful. 

Diamond  Mortar.  A  mortar  made  of  a  piece  of  cylindrical 
tool-steel  about  1  inch  long  and  about  1  inch  in  diameter,  with  a 
convenient  size  cylindrical  cavity  and  pestle  to  fit,  is  very  conven- 
ient for  reducing  a  mineral  to  a  coarse  powder. 

Agate  Mortar.  A  small  agate  mortar,  1>^  inches  in  diameter, 
is  used  for  fine  grinding  of  minerals. 

Steel  Pliers  are  used  for  breaking  off  fragments  of  minerals. 

Platinum  Foil.  A  thin  sheet  of  platinum  about  %  by  %  inch 
may  be  used  for  sodium  carbonate  fusions. 

Dropping  Bulbs  are  useful  for  reagents  that  are  needed  in 
small  amounts,  such  as  cobalt  nitrate  solution. 

Small  beakers,  porcelain  crucibles,  wash-bottles,  etc.,  may 
often  be  used  to  advantage. 

2.  REAGENTS  USED  IN  BLOWPIPE  ANALYSIS 
A.  Dry  Reagents 

Dry  reagents  should  be  kept  in  wide-mouthed  glass  bottles. 
It  is  convenient  to  have  a  set  of  four  to  six  of  these  bottles  in  a 
wooden  stand. 

Sodium  Carbonate,  Na2CO3.  Baking  "  soda  "  (NaHCO3)  may 
be  used  instead.  Sodium  carbonate  is  used  principally  for 
fusions. 

Borax,  Na2B4O7 10B2O,  is  used  principally  for  bead  tests. 
The  ordinary  commercial  salt  may  be  used.  Borax  glass  is 
simply  fused  borax,  used  in  silver  cupellation. 


CHEMICAL  PROPERTIES  OF  MINERALS  23 

Sodium  Metaphosphate,  NaPO3.  This  is  used  for  the  bead 
tests,  in  which  salt  of  phosphorus,  HNaNHdPC^  4H2O,  is  usually 
employed.  It  can  be  made  by  fusing  salt  of  phosphorus,  and 
is  much  more  convenient,  as  a  salt  of  phosphorus  bead  usually 
drops  off  the  loop  of  platinum  wire  when  heated. 

Potassium  Acid  Sulfate,  KBSO4.  This  is  used  in  bismuth 
flux,  in  boric  acid  flux,  and  also  independently. 

lodid  Flux  is  made  by  grinding  together  1  part  KI,  1  part 
KHSO4,  and  2  parts  S.  It  is  used  principally  on  plaster  tablets, 
but  also  on  charcoal. 

Boric  Acid  Flux  is  a  mixture  of  1  part  of  finely  powdered 
fluorite  (CaF2)  with  3  parts  of  KHSO4. 

Cupric  Oxid,  CuO.     Powdered  malachite  may  be  used  instead. 

Tin.  Ordinary  tin-foil  (sheet  lead  with  a  thin  coating  of  tin) 
is  used  as  a  reducing  agent. 

Zinc.  Zinc  in  the  form  of  shavings  or  sheets  is  used  in  test- 
ing cassiterite. 

Test  Lead.  Lead  in  a  granulated  form  such  as  is  used  in 
assaying. 

Bone-Ash,  such  as  is  used  in  assaying,  is  moulded  into  cupels 
on  charcoal.  Prepared  cupels,  1  inch  in  diameter,  may  be  used. 

B.  Wet  Reagents 

The  following  are  the  more  important  wet  reagents  used  in  the 
determination  of  minerals,  though  occasionally  any  of  the 
reagents  of  the  chemical  laboratory  may  be  found  useful. 

Hydrochloric  Acid,  HC1.     Two  parts  concentrated  acid  (sp.  gr.  1.20)  with 

3  parts  distilled  water  is  the  acid  used  for  general  purposes.     (5N.)1 
Nitric  Acid,  HNO3.     One  part  concentrated  acid    (sp.  gr.    1.42)   with 

2  parts  water.     (5N.) 

Sulfuric  Acid,  H2SO4.     One  part  concentrated  acid  (sp.  gr.   1.84)  with 

4  parts  of  water  (5N).     It  should  be  diluted  with  great  care  by  pouring  acid 
into  the  water  rather  than  the  reverse. 

Citric  Acid.  As  this  is  a  solid  it  may  be  used  in  the  field  for  testing 
carbonates.  A  water  solution  is  used. 

1  N  means  a  normal  solution,  i.e.,  one  that  contains  one  gram-equivalent  of  the  sub- 
stance in  one  liter  (a  gram  atom  of  hydrogen  is  the  unit). 


24          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Aqua  Regia  is  a  mixture  of  3  parts  of  cone.  HC1  and  1  part  of  cone.  HNO3. 
It  is  made  up  when  needed. 

Ammonium  Hydroxid,  NH4OH.  One  part  of  concentrated  NH4OH 
(sp.  gr.  0.96)  to  4  parts  of  solution. 

Ammonium  Oxalate,  (NH4)2CO4-2H2O.  40  grams  of  salt  to  a  liter  of 
solution.  (HN.) 

Sodium  Acid  Phosphate,  Na2HPO4-12H2O.  60  grams  to  a  liter  of  solu- 
tion. (KN). 

Ammonium  Molybdate,  (NH4)2MoO4.  This  reagent,  which  is  difficult  to 
prepare,  may  be  made  by  dissolving  100  grams  of  MoO3  in  250  ccm.  NH4OH 
(sp.  gr.  0.96)  with  250  ccm.  of  water.  After  cooling,  this  solution  is  poured 
into  750  ccm.  HNO3  (sp.  gr.  1.2)  with  750  ccm.  water  while  stirring. 

It  may  also  be  prepared  by  dissolving  150  g.  of  (NH4)2MoO4  crystals  in 
a  liter  of  distilled  water  to  which  a  liter  of  HNO3  (sp.  gr.  1.1)  has  been  added. 

Silver  Nitrate,  A3NO3.  43  grams  of  the  salt  to  a  liter  of  solution.  (KN.) 
It  should  be  kept  in  an  opaque  bottle. 

Barium   Chlorid,  BaCl2-2H2O.     61  grams  of  salt  to  a  liter  of  solution. 


Cobalt  Nitrate,  Co(NO3)26H2O.     73  grams  of  the  salt  to  a  liter  of  .solution. 

N.) 

Alcohol.     95  per  cent,  ethyl  alcohol. 

C.  Additional  Reagents  used  in  Qualitative  Analysis 

Acetic  Acid,  HC2H3O2.     30  per  cent.  acid.     (5N.) 

Ammonium  Carbonate,  (NH4)2CO3.  192  grams  to  a  liter  of  solution, 
including  100  ccm.  of  NH4OH.  (4N). 

Ammonium  Chlorid,  NH4C1.  267  grams  to  a  liter  of  solution.  (5N). 
(The  solid  reagent  is  also  used  for  alkali  fusions  with  CaCO3.) 

Ammonium  Sulfid,  (NH4)2S.  Saturate  cone.  NH4OH  with  H2S,  and  add 
an  equal  volume  of  NH4OH.  Dilute  with  three  volumes  of  water.  (4N.) 

Ammonium  Sulfid,  Yellow,  (NH4)2SZ.  This  is  made  by  adding  flowers  of 
sulfur  to  (NH4)2S. 

Barium  Hydroxid,  Ba(OH)2-8H2O.  Used  for  the  detection  of  carbon 
dioxide. 

Calcium  Carbonate,  CaCO3.     The  solid  reagent. 

Chloroplatinic  Acid,  H2PtCl6.  This  is  made  by  dissolving  thoroughly 
cleaned  scrap  platinum  in  aqua  regia. 

Ether  -Alcohol.     Equal  volumes  of  ether  and  absolute  alcohol. 

Dimethylglyoxime.     A  one  per  cent,  solution  of  the  reagent  in  alcohol. 

Ferrous  Sulfate,  FeSO4-7H2O.     Concentrated  solution. 

Lead  Acetate,  Pb  (C2H3O2-3H2O.)     95  grams  to  a  liter  of  solution. 


CHEMICAL  PROPERTIES  OF  MINERALS  25 


Potassium  Chromate,  K2CrO4.     49  grams  to  a  liter  of  solution. 
Potassium  Cyanid,  KCN.     33  grams  to  a  liter  of  solution.     (^N.) 
Potassium  Ferricyanid,  K3Fe(CN)6.     55  grams  to  a  liter  of  solution. 


Potassium  Ferrocyanid,  K4Fe(CN)6-3H2O.  53  grams  to  a  liter  of  solu- 
tion. (KN.) 

Potassium  Hydroxid,  KOH.     Solid  reagent. 

Sodium  Acetate,  NaC2H3O2.     The  solid  dissolved  in  ten  parts  of  water. 

Sodium  Carbonate,  Na2CO3.     Solid  reagent. 

Sodium  Cobaltic  Nitrite,  Na3Co(NO2)6.  This  is  made  by  adding  1  part 
of  Co(NO3)2  solution  to  3  parts  of  acetic  acid  and  5  parts  of  a  10  per  cent. 
solution  of  NaNO2. 

Sodium  Hydroxid,  NaOH.     Solid  reagent. 

Sodium  Nitrate,  NaNO3.     Solid  reagent. 

Stannous  Chlorid,  SnCl2-2H2O.     56  grams  to  a  liter  of  solution.     (3^N.) 

Tartaric  Acid,  C4H4O2.     Solid  reagent. 

3.  THE  OPERATIONS  OF  BLOWPIPE  ANALYSIS 

The  list  of  tests  given  here  serves  both  as  an  outline  to  follow 
with  known  substances,  and 
also  as  determinative  tables 
for  unknown  minerals.  As 
only  the  more  important 
tests  are  included  decided 
results  must  be  obtained 
to  be  of  value. 

FIG.  1.  —  Oxidizing  flame. 

I.  USE  OF  THE  BLOWPIPE. 

To  produce  a  steady  flame,  maintain  a  reservoir  of  air  by 
keeping  the  cheeks  slightly  distended,  and  by  breathing  through 
the  nose. 

Oxidizing  Flame  (O.F.).  The  extreme  outer  tip  (Fig.  1) 
of  a  small  flame  produced  by  a  rather  strong  blast  of  air  is  most 
favorable  for  oxidation.  If  a  candle,  lamp,  or  Bunsen  burner  is 
used,  the  tip  of  the  blowpipe  is  held  just  within  the  flame.  One's 
ability  to  produce  a  good  oxidizing  flame  may  be  judged  by  fusing 
borax  on  a  ^  inch  loop  of  platinum  wire  and  then  adding  a  little 
MoO3.  The  bead  should  become  colorless. 


26          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Reducing  Flame  (R.F.).  The  tip  of  the  inner  luminous  cone 
(Fig.  2)  of  a  large  flame  produced  by  a  gentle  blast  of  air  is  most 
favorable  for  reduction.  If  a  candle,  lamp,  or  Bunsen  burner  is 
used,  the  blowpipe  tip  is  held  just  outside  the  flame  and  the  whole 
flame  is  directed  toward  the  assay.  A  borax  bead  made  ame- 
thyst colored  with  a  little  Mn02  in  O.F.  should  become  colorless 
when  heated  in  a  good  reducing  flame.  The  reducing  flame 
should  be  luminous,  but  just  hot  enough  to  prevent  the  deposi- 
tion of  soot. 

II.  FLAME  TESTS. 

In  the  high  temperature  of  the  blowpipe  flame,  many  com- 
pounds are  volatilized;  and 
the  colors  produced  are  often 
characteristic.  They  should 
be  viewed  against .  a  dark 
background,  such  as  a  piece 
of  charcoal.  The  chlorids, 
as  a  rule,  are  the  most  volatile 
compounds  of  the  metals,  so 

FIG.  2. -Reducing  flame.  HC1  should   be  used>   but  in 

some  cases  H2SO4  is  better. 

Platinum  wire  is  used  except  for  compounds  of  As,  Sb,  Pb,  and 
Cu,  which  should  be  heated  on  charcoal.  The  wire  should  be 
cleaned  with  HC1  after  each  test,  but  it  should  never  be  placed 
in  a  reagent  bottle  on  account  of  the  danger  of  contaminating 
the  reagent. 

Red  Flames. 

Purplish  red — lithium  compounds. 
Crimson — strontium  compounds. 
Orange  red — calcium  compounds. 

Yellow  Flames. 

Intense  yellow  (masked  by  blue  glass  or  flame-color  screen)— 
sodium  compounds. 


CHEMICAL  PROPERTIES  OF  MINERALS  27 

Green  Flames. 

Yellowish  green — barium  compounds. 

Yellowish  green — molybdenum  compounds. 

Emerald  green — copper  compounds  (without  HC1). 

Bright  green  (use  H2SO4) — boron  compounds. 

Pale  bluish  green  (use  H2SO4) — phosphates. 

Pale  bluish  green — tellurium  compounds. 

Pale  bluish  green — antimony  compounds. 

Bluish  green — zinc  compounds. 
Blue  Flames. 

Azure  blue — copper  compounds  (with  HC1). 

Pale  blue — arsenic  compounds. 

Pale  blue — lead  compounds. 
Violet  Flames. 

Pale  reddish  violet  (use  blue  glass  or  flame-color  screen) 
— potassium  compounds.  (Some  potassium  compounds  such  as 
orthoclase  must  be  fused  with  Na2CO3,  to  obtain  the  flame  test.) 
The  spectroscope  must  be  used  to  detect  such  elements  as 
rubidium,  calcium,  thallium,  indium,  etc.,  and  also  to  detect  very 
small  amounts  of  the  above  mentioned  elements. 

III.  OPEN  TUBE  TESTS. 

Glass  tubes  about  4  or  5  inches  long  and  open  at  both  ends  are 
used.  The  substance,  which  may  be  introduced  into  the  tube  by 
means  of  a  folded  trough  of  paper,  is  placed  about  1  inch  from 
one  end  of  the  tube.  The  tube  is  heated  gently  in  a  horizontal 
position  at  first  and  then  is  gradually  inclined  while  still  heating; 
thus  a  current  of  air  is  produced.  If  heated  too  rapidly  or  too 
near  the  end  of  the  tube  a  closed  tube  effect  is  the  result. 

Odor  of  burning  matches   (S02) — sulfids  and  sulfo-salts. 

Sublimate   of  minute  brilliant  crystals   (As2O3) — arsenids 
and  sulfarsenites. 

Non-volatile  amorphous  sublimate  (Sb2O4)  on  under  side  of 
tube — antimony  sulfid  and  sulfantimonites. 

Gray  metallic  globules  (Hg) — mercury  sulfid. 


28          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

IV.  CLOSED  TUBE  TESTS. 

Glass  tubes  closed  at  one  end  are  used.  Two  closed  tubes  may 
be  made  at  the  same  time  by  fusing  a  piece  of  tubing  5  or  6  inches 
long  at  its  middle  point  and  pulling  it  apart  when  hot.  Tubes 
should  be  clean  and  dry  before  they  are  used. 

1.  Change  in  Appearance. 

Decrepitates  (flies  to  pieces) — characteristic  of  many 
minerals. 

Turns  black — copper  minerals  and  numerals  containing 
organic  matter. 

Turns  dark  red — iron  minerals. 

Turns  yellow — lead  minerals. 

Turns  yellow  (white  on  cooling) — zinc  minerals. 

2.  Formation  of  Sublimates. 

Yellow  sublimate  (S) — some  sulfids. 
Black  metallic  mirror  (As) — arsenids. 
Reddish  yellow   (AsS) — arsenic  sulfids  and  sulfarsenites. 
Reddish  brown  (Sb2S2O) — antimony  sulfids  and  sulfanti- 
monites. 

White  volatile  sublimate — ammonium  salts. 

Water  (H20) — hydroxids,  hydrous,  basic,  and  acid  salts. 

3.  Formation  of  Gases. 

Colorless  and  odorless  (C02) — carbonates  (detected  by 
Ba(OH)2. 

Colorless  and  odorless  (O) — manganese  dioxids  (detected  by 
glowing  charcoal). 

Brownish-red  and  pungent  odor  (NO2) — nitrates. 

V.  TREATMENT  ON  CHARCOAL. 

The  substance,  either  alone  or  intimately  mixed  with  some 
reagent,  is  heated  in  a  shallow  circular  cavity  at  one  end  of  the 
charcoal,  which  is  made  by  revolving  a  coin  or  end  of  a  knife 
handle.  O.F.  or  R.F.  is  used  according  to  the  desired  effect. 


CHEMICAL  PROPERTIES  OF  MINERALS  29 

1.  Evolution  of  Gas. 

Odor   of   burning   matches    (SO2) — sulfids  and  sulfo-salts 
(use  O.F.). 
Arsin  odor  (AsH3) — arsenids  and  sulfarsenites  (use  R.F.). 

2.  Formation  of  Sublimates.     (Use  O.F.). 

It  is  well  to  run  a  blank  test  to  observe  ash  of  the  charcoal. 
White  sublimate  near  assay  (Sb203) — antimony  compounds. 
White  sublimate  far  from  assay  (As2O3) — arsenic  compounds. 
White  sublimate,  yellow  when  hot  (ZnO) — zinc  compounds. 
White  sublimate,  yellow  near  assay  (PbSO4) — lead  sulfid. 
Yellow  sublimate  (PbO) — lead  compounds. 
Yellow  sublimate  (Bi2O3) — bismuth  compounds. 

3.  Reduction  with  Sodium  Carbonate. 

Mix  intimately  1  part  of  the  finely  powdered  substance  with 
3  parts  of  Na2CO3  and  fuse  in  R.F.  on  charcoal. 

Magnetic  particles  (Fe3O4,Ni,Co) — iron,  nickel,  and  cobalt 
compounds. 

Metallic  button,  gray  and  malleable  (Pb) — lead  compounds. 

Metallic  button,  somewhat  malleable  but  brittle  on  edges 
(Bi) — bismuth  compounds. 

Metallic  button,  malleable  white  (Ag) — silver  compounds. 

Metallic  button,  malleable  yellow  (An) — gold  compounds. 

Metallic  button,  malleable  red  (Cu) — copper  compounds. 

Metallic  button,  malleable  white  (Sn) — tin  compounds. 

4.  Fusion  Test  for  Sulfur. 

An  intimate  mixture  of  a  finely  powdered  sulfid  or  sulfo-salt 
with  about  three  parts  of  sodium  carbonate  is  heated  in  O.F.  on  a 
thin  sheet  of  mica  placed  on  charcoal  (or  on  platinum  foil  if 
absence  of  As,  Sb,  Pb,  and  Cu  is  assured).  The  fused  mass  placed 
on  a  bright  silver  coin  with  several  drops  of  water  and  crushed 
will  give  a  black  stain  (Ag2S).  The  reactions  are:  R"S  +  Na2- 
CO3  =  Na2S  +  R"CO3.  Na2S  +  2Ag  +  H2O  +  O  =  Ag2S  + 
2NaOH.  Tellurids  give  the  same  test  as  sulfids. 


30          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

It  is  well  to  run  a  blank  test  to  see  if  the  gas  or  sodium  car- 
bonate contains  sulfur. 

Sulfates  also  give  this  test  if  heated  in  a  strong  R.F.  on  char- 
coal instead  of  on  mica.  The  addition  of  powdered  charcoal 
helps  the  reaction. 

As   the  sulfur  compounds  sink  into  the  charcoal,  the  same 
spot  cannot  be  used  more  than  once. 
5.  Treatment  with  Cobalt  Nitrate. 

The  substance  is  heated  intensely  on  charcoal  before  and 
after  adding  a  dilute  solution  of  cobalt  nitrate.  In  this  way 
cobalt  aluminate,  cobalt  zincate,  etc.,  are  formed. 

Deep  blue  coloration — infusible  aluminum  compounds  and 
zinc  silicates.  (Almost  any  fusible  substance  will  give  a  blue 
color,  for  a  cobalt  glass  is  formed.) 

Bright  green  coloration — zinc  compounds,  except  the  sili- 
cates which  give  a  blue  coloration. 

Bluish-green  coloration — tin  compounds. 

Pale  pink  coloration — magnesium  compounds  (not  very 
satisfactory). 

VI.  TREATMENT  ON  PLASTER  WITH  IODID  FLUX. 

An  intimate  mixture  of  the  substance  with  an  equal  quantity 
of  iodid  flux  (2  parts  S,  1  part  KI,  .and  1  part  KHS04)  is  heated 
gently  at  one  end  of  a  plaster  tablet.  In  this  way  iodids  of 
the  metals  are  obtained. 

Yellow  sublimate  (PbI2) — lead  compounds. 

Orange  sublimate  stippled  with  peach-red  (SbI3) — antimony 
compounds. 

Purplish-chocolate  sublimate  (BiI3)  with  underlying  scarlet 
— bismuth  compounds. 

Scarlet  sublimate  (dark  greenish-yellow  if  overheated)  (HgI2)— 
mercury  compounds. 

VII.  TREATMENT  ON  CHARCOAL  WITH  IODID  FLUX. 

The  same  reagent  as  above,  but  with  charcoal  instead  of  plaster 
as  a  support. 


CHEMICAL  PROPERTIES  OF  MINERALS 


31 


Greenish-yellow  sublimate  (Pbl2) — lead  compounds. 
Scarlet  sublimates  (BiI3) — bismuth  compounds. 
Faint  yellow  sublimate  (HgI2,  etc.) — mercury,  arsenic,  and 
antimony  compounds. 

VIII.  BORAX  BEAD  TESTS. 

Borax  beads  are  made  by  fusing  borax  in  a  3  mm.  loop  of 
platinum  wire  formed  around  the  sharpened  end  of  a  lead  pencil. 
Great  care  should  be  used  in  O.F.  and  R.F.  Sulfids  should  be 
first  roasted  by  gently  heating  the  powdered  substance  spread 
out  on  charcoal.  It  is  well  to  preserve  the  beads  in  a  little 
frame  or  glass  tube  for  future  reference.  The  colors  refer  to 
cold  beads,  except  when  otherwise  mentioned.  Many  elements 
giving  colorless  or  pale  yellow  beads  are  not  mentioned. 


Violet 

Blue 

Green 

Red 

Brown 

Yellow 

Colorless 

Co. 

O.F.,R.F. 

Cr 

O.F.,R.F. 

Cu 

O.F. 

O.F.,R.F. 

(saturated) 

R.F. 

(opaque) 

Fe 

R.F. 

O.F. 

Mn 

O.F. 

R.F. 

Mo 

R.F. 

O.F. 

Ni 

O.F. 

(hot) 

O.F. 

(cold) 

R.F. 

(turbid  gray) 

Ti 

R.F. 

O.F. 

U 

R.F. 

O.F. 

V 

R.F. 

O.F. 

W 

R.F. 

O.F. 

32          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

IX.  SODIUM  METAPHOSPHATE  BEAD  TESTS. 

Beads  of  sodium  metaphosphate,  NaP03,  are  made  in  the 
same  way  as  with  borax.  Salt  of  phosphorus  or  microcosmic  salt, 
(HNaNH4PO4-4H2O)  may  also  be  used,  for  on  heating  it  loses 
NH3  and  H2O,  and  is  converted  into  NaP03.  The  colors  refer 
to  cold  beads. 


Violet 

Blue 

Green 

Red 

Yellow 

Colorless 

Co 

O.F.,R.F. 

Cr 

O.F.,R.F. 

Cu 

O.F. 

(satur- 
ated) 

R.F. 

(opaque) 

Fe 

O.F. 

(pale) 

O.F.,R.F. 

Mn 

O.F. 

R.F. 

Mo 

R.F. 

O.F. 

Ni 

O.F.,R.F. 

Ti 

(satur- 
ated) 

R.  F. 

O.F. 

U 

O.F.,R.F. 

V 

R.F. 

O.F. 

W 

R.F. 

O.F. 

Silica  is  insoluble  in  a  NaPO3  bead,  but  with  silicates  the 
beads  dissolve  (sometimes  coloring  the  bead),  while  the  silica 
usually  remains  as  a  translucent  mass,  often  the  shape  of  the 
original  fragment,  which  floats  around  in  the  bead.  A  few 
other  compounds,  such  as  A12O3  and  Ti02,  are  very  slowly  soluble 
in  a  NaP03  bead. 


CHEMICAL  PROPERTIES  OF  MINERALS  33 

X.  REDUCTION  COLOR  TESTS. 

Saturate  several  NaPO3  beads  with  the  finely  ground  sub- 
stance, and  heat  on  charcoal  with  metallic  tin  in  R.F.  Dissolve 
in  dilute  HC1,  add  tin,  and  then  boil. 

Violet  solution — titanium  compounds. 

Deep  blue  solution — tungsten  compounds. 

Brown  solution — molybdenum  compounds. 

Green  solution — chromium,  uranium,  and  vanadium  com- 
pounds. 

XI.  SODIUM  CARBONATE  BEAD  TEST. 

Beads  of  sodium  carbonate  are  made  the  same  as  borax  and 
sodium  metaphosphate.  The  O.F.  is  used.  The  beads  are 
opaque  and  not  clear  as  with  borax. 

Bluish-green  opaque  bead   (Na2MnO4) — manganese   com- 
pounds (a  very  delicate  test). 

Yellow   opaque   bead    (Na2CrO4) — chromium    compounds. 
Effervescence— silica.     Na2CO3  +   SiO2  =  Na2SiO3  + 
CO2.     (The  bead  will  be  clear  if  equal  molecular  quantities 
•  are  used.) 

XII.  TREATMENT  WITH  ACID  POTASSIUM  SULFATE. 

The  substance  is  mixed  with  KHSO4  and  heated  in  a  test-tube 
or  closed  tube. 

Red-brown  fumes  with  pungent  odor  (NO2) — nitrates. 
Colorless  gas  with  HC1  odor  (HC1) — chlorids. 
Colorless  gas  which  etches  glass  (HF) — fluorids. 
Colorless  gas  with  disagreeable  odor  (H2S) — sulfids. 
Colorless,  odorless  gas  (C02) — carbonates. 

XIII.  FUSIBILITY  TESTS. 

Long  thin  splinters  of  the  mineral  about  1  mm.  in  diameter 
held  with  platinum-tipped  forceps  or  wrapped  with  a  coil  of 
platinum  wire  (in  the  absence  of  platinum  forceps  or  wire  a 
splinter  may  be  stuck  into  a  piece  of  charcoal)  are  heated  in  the 
hottest  part  of  the  flame,  which  is  just  beyond  the  tip  of  the  inner 


34          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

cone  (see  Fig.  3)  of  a  small  sharp  O.F.  flame  (rather  strong 
blast).  Metallic  substances  should  be  heated  on  charcoal  as 
they  may  contain  As,  Sb,  or  Pb  which  will  alloy  with  the  plati- 
num. Powders  or  substances  which  fly  to  pieces  when  heated 
may  be  ground  with  a  little  water  into  a  paste,  which  after 
careful  drying  can  be  heated  in  the  forceps  or  on  charcoal. 

Scale  of  Fusibility 

1.  Fuses  easily  in  luminous  flame  or  in  the  closed  tube — 

stibnite. 

2.  Fuses  with  difficulty 
in  luminous  flame  or  in  the 
closed  tube — chalcopyrite. 

3.  Fuses  easily  in  blow- 
pipe flame — almandite 

FIG.  3. — Position  of  assay  in  testing  fusibility.    ^*    •  ... 

4.  Fuses  on  edges  easily 
in  blowpipe  flame — actinolite. 

5.  Fuses  on  edges  with  difficulty — orthoclase. 

6.  Fuses  only  on  thinnest  edges — enstatite. 

7.  Infusible,  even  on  thinnest  edges — quartz. 

Not  only  the  degree,  but  also  the  manner  of  fusion  should  be 
noted.  The  substance  may  fuse  either  to  a  clear,  opaque,  or 
colored  glass;  quietly,  with  intumescence  (bubbling),  or  with 
exfoliation  (spreading  out  like  leaves  of  a  book). 

XIV.  BLOWPIPE  SILVER  ASSAY. 

A  qualitative  test  for  silver  in  ores  may  easily  be  carried  out  by 
means  of  the  blowpipe.  The  method  is  similar  to  that  used  in 
assaying  except  that  it  is  on  a  smaller  scale. 

By  using  an  assay  centner  (100  mg.)  of  ore  and  measuring  the 
silver  button  obtained  on  an  ivory  scale  made  for  the  purpose, 
one  may  obtain  quantitative  results  which,  after  some  practice, 
are  very  satisfactory. 

(1)  Mix  finely  powdered  ore  intimately  with  one  volume  of 


CHEMICAL  PROPERTIES  OF  MINERALS  35 

borax  glass  (made  by  fusing  borax)  and  one  volume  of  test  lead. 
If  ore  contains  galena  it  is  not  necessary  to  add  test  lead.  (2) 
Fuse  mixture  in  a  deep  cavity  in  charcoal  with  a  strong  R.F.  for 
several  minutes.  The  silver  is  collected  by  the  lead  button. 
(3)  After  cooling  remove  lead  from  charcoal  and  hammer  off  the 
slag.  (4)  Add  fresh  borax  glass  and  heat  in  O.F.  until  the 
quantity  of  lead  is  considerably  diminished.  Again  hammer 
off  every  particle  of  the  slag.  (5)  Prepare  a  cupel  by  rilling  a 
large  cavity  in  charcoal  with  very  slightly  moistened  bone-ash  and 
making  a  smooth  concave  depression  with  a  mold  (the  end  of  a 
large  test-tube  will  do).  Heat  cupel  gently  and  remove  all  loose 
particles.  (6)  Carefully  place  the  cube  of  lead  (which  should  be 
not  more  than  2  or  3  mm.  in  diameter)  on  the  cupel  and  fuse  in 
O.F.  by  blowing  across  the  top  of  it  (use  a  small  flame  and 
strong  blast  and  revolve  the  cupel  occasionally) .  The  oxidation 
produces  a  thin  film  of  lead  oxid  showing  interference  colors,  but 
when  the  lead  is  all  absorbed,  the  film  suddenly  disappears  or 
"blicks,"  and  a  minute  sphere  of  silver,  which  may  also  contain 
gold,  remains.  The  final  oxidation  of  the  lead  must  proceed 
without  interruption,  otherwise  it  may  be  necessary  to  repeat 
the  entire  operation  from  the  beginning. 

If  the  button  shows  a  yellow  tinge,  gold  is  present.  The  silver 
may  be  removed  by  dissolving  the  button  in  nitric  acid,  but  if 
much  gold  is  present  it  is  necessary  to  add  some  silver  to  the 
button  in  order  to  separate  the  gold  from  the  silver. 

XV.  SOLUBILITY  TESTS. 

In  the  absence  of  any  special  phenomena  such  as  the  evolution 
of  a  gas,  or  change  in  color,  the  only  accurate  way  of  testing 
solubility  is  to  boil  a  small  amount  of  the  solvent  with  the  sub- 
stance for  some  time,  and  then  to  filter  or  decant  the  clear  liquid 
and  evaporate  it  to  dryness.  A  residue  indicates  that  the  sub- 
stance is  soluble  (anhydrite  furnishes  a  good  example  of  a  soluble 
mineral  which  on  hasty  examination  one  might  call  insoluble). 
If  in  doubt  as  to  the  solubility  run  a  blank  test  with  an  equal 


36          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

quantity  of  solvent  alone.  A  water  solution  of  the  residue  gives 
a  precipitate  with  a  solution  of  sodium  carbonate,  except  in  the 
case  of  alkali  compounds,  but  among  minerals  these  are  all  readily 
soluble  in  water  or  contain  other  elements  that  are  precipitated. 

Soluble  in  water — nitrates,  some  chlorids,  some  sulfates,  some 
borates,  some  carbonates. 

Soluble  in  HC1 — all  carbonates,  some  sulfids,  some  sulfates, 
borates,  some  phosphates,  some  silicates  (see  p.  495),  iron  oxids, 
and  iron  hydroxids. 

Soluble  in  HN03,  but  insoluble  in  HC1 — most  sulfids  and  sulfo- 
salts. 

Soluble  in  aqua  regia — gold  and  platinum. 

Soluble  in  HF — silica  and  nearly  all  the  silicates. 

Insoluble  in  acids  but  soluble  in  other  liquids — cerargyrite, 
soluble  in  NH4OH;  anglesite,  soluble  in  NH4(C2H3O2);  sulfur, 
soluble  in  CS2. 

Insoluble,  but  decomposed  by  fusion  with  Na2CO3 — most 
silicates,  chromite,  wolframite,  barite,  and  celestite.  For 
method  of  treatment  see  note  4,  for  silicates,  p.  49. 

Insoluble,  not  completely  decomposed  by  Na2CO3  fusion,  but 
decomposed  by  fusion  with  KOH  in  a  nickel  crucible — cassiterite, 
corundum,  and  rutile. 

Evolution  of  Gas. 

Colorless,  odorless  gas  (CO2) — carbonates. 

Colorless  gas  with  disagreeable   odor   (H2S) — some   sulfids. 

Colorless,   pungent  gas   (Cl)   with  HC1 — manganese  dioxid. 

Brown  red,  pungent  gas  (NO2)  with  HN03 — sulfids  and  some 
elements. 

Color  of  Solution. 

Amber  solution — iron  compounds. 

Green  solution — copper  (especially  when  iron  is  present) 
and  nickel  compounds. 

Blue  solution — copper  compounds. 

Pale  red  solution — cobalt  compounds. 


CHEMICAL  PROPERTIES  OF  MINERALS  37 

Insoluble  Residue. 

Gelatinous  residue  or  slimy  silica — some  silicates. 

White  residue  (PbSO4),  (HSbO3),  (AgCl)— lead,  antimony, 
and  silver  minerals. 

Yellow  residue  (WO8) — calcium  tungstate. 

XVI.  WET  TESTS  AND  GROUP -REAGENTS. 

A.  Wet  Tests  for  Metals  (Cations) 

HC1  precipitates  AgCl,  HgCl,  and  PbCl2. 

H2S  in  acid  solutions  precipitates  Ag2S,  PbS,  HgS  +  Hg,  Bi2S3, 
CuS,  HgS,  As,S3,  As2S3  +  S,  Sb2S3,  SnS,  and  SnS2. 

NH4OH  in  the  presence  of  HC1  (or  NH4C1)  precipitates 
Pb(OH)2,  Hg2NH2Cl,  HgNH2Cl,BiO(OH),SbO(OH),Sn(OH)2r 
Sn(OH)2,  Al(OH),,  Cr(OH)3;  Fe(OH)3,  Fe(OH)2,  and  also  Ca3- 
(PO4)2,  CaF2,  and  Ca(BO2)2. 

(NH4)2S  in  neutral  solutions  precipitates  Ag2S,  PbS,  HgS, 
CuS,  Bi2S3,  Sb2S3,  SnS,  Al(OH),,  Cr(OH3),  FeS,  FeS  +  S,  ZnS, 
MnS',  CoS,  and  NiS. 

(NH4)2CO3  precipitates,  from  alkaline  solutions,  carbonates  of 
all  the  non-alkali  metals  except  Mg.  With  Ag,  Cu,  Co,  Ni,  and 
Zn  the  precipitate  is  soluble  in  excess. 

Na2HPO4  precipitates  all  the  metals  except  the  alkalies  as 
phosphates,  Hg  as  basic  chlorid,  and  Sb  as  oxid. 

Na2CO  3  precipitates  all  the  metals  except  the  alkalies  as  follows : 
Ag2CO3,Hg2CO3,CdCO3,FeCO3,MnC03,BaCO3,  SrC03.  CaCO3, 
MgCO3,  Fe(OH)3,  Al(OH),,  Cr(OH)3,  Sn(OH)2,  H2Sn03,  Sb2O3, 
Hg2OCl2,  H3SbO4,  and  basic  carbonates  of  Pb,  Cu,  Zn,  Co,  andNi. 

H2SO4  (dilute)  precipitates  PbS04,  BaSO4,  SrS04,  CaSO4-2H2O 
(incompletely  unless  alcohol  is  added),  and  HgSO4  (incompletely). 

NaOH  precipitates  Ag2O,  Hg2O,  HgO,  Cu(OH)2,  Cd(OH)2, 
BiO(OH),  SbO(OH)3,  Sn(OH)2,  SnO(OH)2,  Fe(OH)3,  Fe(OH)2, 
Ni(OH)2,  Co(OH)2,  Mn(OH)2,  Ba(OH)2  (incompletely),  Sr(OH)2 
(incompletely),  Ca(OH)2  (incompletely),  Mg(OH)2,  and  the  fol- 
lowing which  are  soluble  in  excess:  Pb(OH)2,  Sb2O3,  SbO(OH)3, 
Sn(OH)2,  SnO(OH)2,  Al(OH),,  Cr(OH)3,  Zn(OH)2. 


38          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

(NH4)2C2O4  precipitates  oxalates  of  all  the  metals  except  the 
alkalies  and  magnesium  from  alkaline  solutions. 

B.  Wet  Tests  for  Acid  Radicals  (Anions) 

With  BaCl2  as  a  reagent. 

A  white  ppt.  insoluble  in  HC1  indicates  864. 

A  white  ppt.  soluble  in  HCl,  but  insoluble  in  acetic  acid  indi- 
cates F. 

A  yellow  ppt.  soluble  in  HCl  but  insoluble  in  acetic  acid 
indicates  CrC>4. 

A  white  ppt.  soluble  in  HCl  and  in  acetic  acid  indicates  BO2 
or  B4O7,  PO4,  CO 3,  or  AsO4. 
With  AgNO  3  as  a  reagent. 

A  yellow  ppt.  soluble  in  HNO3  indicates  PO4. 

A  red  or  red-brown  ppt.  soluble  in  HN03  indicates  AsO4or  CrO4. 

A  white  ppt.  soluble  in  HN03  indicates  BO2  or  640?. 

A  white  ppt.  insoluble  in  HNO3  indicates  Cl. 

A  black  ppt.  soluble  in  HN03  indicates  S. 

XVII.  PREPARATION  OF  SOLUTION. 

Water  is  the  first  solvent  used,  and  after  that  either  hydro- 
chloric or  nitric  acids.  For  some  minerals  HCl  is  the  best  solvent 
and  for  some  HNO3  is  the  best;  therefore  it  is  well  to  try  a  small 
quantity  of  the  mineral  with  each  of  these  solvents  to  determine 
which  is  the  better.  For  sulfids  HN03  is  the  best  solvent,  but  if 
either  lead,  antimony,  or  tin  is  present,  white  residues  are  formed. 
HCl  will  precipitate  chlorids  of  silver,  lead,  and  mercury.  If 
the  substance  is  insoluble  in  both  HNO3  and  HCl  it  may  be 
soluble  in  aqua  regia  (1  part  HNO3+3  parts  HCl). 

Many  minerals,  especially  silicates,  are  insoluble  in  aqua  regia, 
and  require  fusion  with  Na2CO3  on  platinum  foil  or  in  a  porcelain 
crucible.  A  water  solution  of  the  fusion  will  generally  contain 
sodium  salts  of  various  acids,  while  an  acid  solution  of  the  residue 
will  generally  contain  the  metals. 

The  following  minerals  are  not  decomposed  by  Na2CO3  and 


CHEMICAL  PROPERTIES  OF  MINERALS  39 

require  fusion  with  KOH  in  a  nickel  or  silver  crucible :  corundum, 
(AlaO,),  cassiterite,  (Sn02),  and  rutile,  (TiO2). 

XVIII.  QUALITATIVE   SCHEME.     (For   the   more    common   ele- 
ments) . 

1.  Add  cold  dilute  HC1  in  excess.     Ppt.  2.     Filtrate  6. 

2.  Wash  ppt.  with  hot  water  on  filter-paper.     Residue  3.     Filtrate  5. 

3.  Add  NH4OH  to  residue  drop  by  drop.     A  blackening  indicates  Hg. 
Divide  nitrate  into  two  portions  4  and  5. 

4.  Acidify  filtrate  with  HNO3.     A  white  ppt.  indicates  Ag. 

5.  Test  filtrate  with  K2CrO4.     A  yellow  ppt.  indicates  Pb. 

6.  Pass  H2S  into  warm,  slightly  acid  solution.     Ppt.  7.     Filtrate  16. 

7.  Digest  ppt.  with  (NH4)2S.     Filter.     Residue  8.     Filtrate  13. 

8.  Digest    residue    with    hot    dilute    HNO3.     Filter.     Residue    9. 
Filtrate  10. 

9.  Dissolve  residue  in  aqua  regia.     Boil  off  Cl.     A  ppt.  with  SnCl2 
indicates  Hg. 

10.  Add  a  little  cone.  H2SO4  and  drive  off  excess.     A  white  ppt. 
indicates  Pb.     Filtrate  11. 

11.  Add  NH4OH  in  excess  to  filtrate.     A  white  ppt.  indicates  Bi. 
Filtrate  12. 

12.  A  blue  filtrate  indicates  Cu.     Add  KCN  until  blue  color  disap- 
pears; then  pass  H2S.     A  yellow  ppt.  indicates  Cd. 

13.  Add  dilute  HC1  to  filtrate.     Heat  ppt.  formed  with  cone.  HC1. 
A  residue  indicates  As.     Filtrate  14. 

14.  Into  the  dilute  solution,  heated  to  almost  boiling,  pass  H2S. 
An  orange  red  ppt.  indicates  Sb.     Filtrate  15. 

15.  Into  the  cool  diluted  filtrate  pass  H2S.     A  yellow  ppt.  indicates 
Sn. 

16.  Boil  off  H2S,  add  a  few  drops  of  HNO3.     Add  NH4C1  and 
NH4OH.     Ppt.  17.     Filtrate  22. 

17.  Dissolve  ppt.  in  least  possible  amount  of  HC1.     Add  50  %  alco- 
hol and  dilute  H2SO4.     A  crystalline  ppt.  indicates  Ca.     Filtrate  18. 

18.  Boil  off  the  alcohol,  make  filtrate  alkaline  with  NH4OH.     Ppt. 
19.     Reject  filtrate. 

19.  Fuse  ppt.  with  Na2CO3  and  NaNO3  on  platinum  foil.     A  bluish- 
green  mass  indicates  Mn.    Digest  the  fused  mass  in  hot  water  and  filter. 
Residue  indicates  Fe.     Divide  filtrate  into  two  portions,  20  and  21. 

20.  A  yellow  filtrate  giving  red  ppt.  with  AgNO3  indicates  Cr. 

21.  Acidify  with  HC1.     Add  solid  NH4C1  and  boil.     A  ppt.  indi- 
cates Al. 


40          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

22.  Into  the  warm  alkaline  filtrate  pass  H2S.     Ppt.  23.     Filtrate  29. 

23.  Wash  ppt.  on  filter  with  cold  dilute  (1  :  10)  HC1.     Residue  24. 
Filtrate  26. 

24.  Dissolve  residue  in  aqua  regia.     Evaporate  to  dryness,  add  a 
little  water,  and  make  strongly  basic  with  NaOH.     Add  tartaric  acid 
but  not  enough  to  make  the  solution  acid.     Heat  slightly  and  pass 


a 
1 

3 

w 
& 


2 
O 


H2S.     A  ppt.  indicates  Co.     Filtrate  25. 

25.  Acidify  filtrate  with  HC1.     A  ppt.  indicates  Ni. 

26.  Boil  filtrate  to  remove  H2S.     Add  KOH  in  excess.     Ppt.  27. 
Filtrate  28. 

27.  Fuse  ppt.  with  Na2CO3.     A  bluish-green  mass  indicates  Mn. 

28.  Add  H2S  to  the  filtrate  and  heat.     A  white  ppt.  indicates  Zn. 

29.  Evaporate  filtrate  to  rather  small  volume.     Add  (NH4)2CO3 
and  alcohol.     After  standing  an  half-hour,  filter.     Ppt.  30.     Filtrate 
34. 

30.  Dissolve  ppt.  in  hot  dilute  acetic  acid  and  add  K2CrO4.     A 
yellow  ppt.  indicates  Ba.     Filtrate  31. 

31.  Add  NH4OH  and  alcohol.     A  yellow  ppt.  indicates  Sr.     Fil- 
trate 32. 

32.  Dilute  and  add  (NH4)2C2O4.     A  white  ppt.  indicates  Ca.     Fil- 
trate 33. 

33.  Add  NH4OH  and  Na2HPO4.     A  white  ppt.  indicates  Mg. 

34.  Evaporate  filtrate  to  dryness.     Ignite  to  drive  off  ammonium 
salts.     Add  NaOH  and  Na2HPO4.     Heat  and  add  alcohol.     A  white 
ppt.  indicates  Li.     Filtrate  5. 

35.  To  the  filtrate  add  Na3Co(NO2)6.     A  yellow  ppt.  indicates  K. 
Note. — The  original  substance  must  be  tested  for  Na  and  NH4. 

4.  SELECT  BLOWPIPE  AND  WET  TESTS 
Aluminum,  Al. 

1.  Infusible   aluminum   minerals    (also   zinc   silicates)    when 
heated  intensely  before  and  after  adding  cobalt  nitrate  solution 
give  a  fine  blue  color.     Fusible  minerals  may  give  a  blue  cobalt 
glass  whether  aluminum  is  present  or  not. 

2.  Ammonium  hydroxid  gives  a  white  gelatinous  precipitate, 
A1(OH)3,  in  solutions  containing  aluminum.     (The  ppt.  is  soluble 
in  KOH  or  NaOH.)     Iron  hydroxid,  chromium  hydroxid,  cal- 
cium phosphate,  calcium  borate,  and  calcium  fluorid  are  also 
precipitated  by  NH4OH  along  with  A1(OH)3.     The  calcium  may 


CHEMICAL  PROPERTIES  OF  MINERALS  41 

be  removed  by  means  of  dilute  sulfuric  acid  and  50  per  cent, 
ethyl  alcohol  before  testing  for  iron  and  aluminum. 

Antimony,  Sb. 

1.  Antimony  minerals  heated  on  charcoal  in  O.F.  give  a  volatile 
white  sublimate  (Sb2C>4)  near  the  assay  and  dense  white  fumes 
without  odor. 

2.  With  iodid  flux  on  a  plaster  tablet  antimony  compounds  give 
a  peach-red  coating  or  an  orange  coating  stippled  with  peach-red. 

3.  In  the  open  tube  antimony  minerals  give  a  non- volatile, 
amorphous,  white  sublimate  (Sb204)  on  the  under  side  of  the  tube. 

4.  Compounds  of  antimony  and  sulfur  give  a  reddish-brown 
sublimate  (Sb2S20)  when  heated  intensely  in  the  closed  tube. 

5.  Concentrated  HN03  oxidizes  antimony  sulfids  and  sulfo- 
salts  to  HSb03,  a  white  precipitate  soluble  in  KOH. 

Arsenic,  As. 

A.  Compounds  without  Oxygen. 

1.  On  charcoal  most  arsenic  minerals  give  a  white  volatile 
coating  (As2O3)  far  from  the  assay  and  fumes  with  characteristic 
odor  of  arsin  (AsH3)  (a  disagreeable  odor  something  like  that  of 
garlic) . 

2.  In  the  open  tube,  minute,  brilliant,  colorless  crystals  (As2- 
O3).     This  sublimate  is  volatile  in  contrast  with  that  of  Sb2O4. 

3.  In  the  closed  tube  a  black  metallic  mirror  of  arsenic.     A 
gray  crystalline  sublimate  may  also  form. 

4.  H2S  precipitates  yellow  As2S3,  which  is  soluble  in  (NH4)2- 
Sx,  but  insoluble  in  concentrated  HC1. 

B.  Ar senates. 

5.  Arsenates  heated  intensely  in  the  closed  tube  with  charcoal 
give  a  black  metallic  mirror  of  arsenic. 

6.  Nitric  acid  solutions  of  arsenates  give  a  yellow  precipitate 
with  (NH4)2MoO4  when  heated  to  boiling. 

If  the  solution  is  to  be  tested  for  a  phosphate,  the  arsenic 
must  be  removed  by  means  of  H2S  (see  4). 


42          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Barium,  Ba. 

1.  Yellowish-green  flame  (not  made  blue  by  HC1). 

2.  Dilute  H2S04  precipitates  white  BaSO4,  a  finely  divided 
precipitate   insoluble   in   acids.     This,   like   strontium   sulfate, 
forms  in  very  dilute  solutions  while  calcium  sulfate  forms  only  in 
fairly  concentrated  solutions. 

3.  (NH4)2CO3[or  (NH4)2C204]  gives  a  white  precipitate  soluble 
in  acids.     (Sr  and  Ca  also.) 

4.  K2Cr04     (or  K2Cr207)    gives    a    yellow    precipitate    (dis- 
tinction from  Sr  and  Ca). 

Beryllium,  Be. 

1.  Be(OH)2  is  precipitated  along  with  A1(OH)3  by  NH4OH. 
The  precipitate  is  dissolved  in  dilute  HC1  and  the  solution 
evaporated  nearly  to  dryness.  A  little  water  is  added,  and  also 
KOH  in  amount  sufficient  to  dissolve  the  precipitate  which  forms 
at  first.  The  solution  is  diluted  and  boiled  when  Be(OH)2 
separates  out.  This  precipitate  heated  on  charcoal  with  cobalt 
nitrate  solution  assumes  a  lavender  color. 

Bismuth,  Bi. 

1.  With  iodid  flux  on  plaster,  a  purplish-chocolate  sublimate 
with  underlying  scarlet. 

2.  With  sodium  carbonate  on  charcoal  in  R.F.,  a  metallic 
button  brittle  on  the  edges,  and  also  a  yellow  sublimate  (BiO). 

3.  To  a  nitric  acid  solution  from  which  the  excess  of  acid  has 
been  evaporated,  HC1  is  added.     On  dilution  with  water,  a  white 
precipitate  of  bismuth  oxychlorid  (BiOCl)  is  formed. 

Boron,  B. 

1.  Borates  give  a  green  flame,  especially  if  moistened  with 
H2S04.  Silicates  containing  boron  give  a  momentary  green 
flame  when  heated  with  boric  acid  flux  (3  parts  KHSO4  to  1 
part  powdered  fluorite,  CaF2) .  This  flame  is  due  to  the  forma- 
tion of  volatile  BF8. 


CHEMICAL  PROPERTIES  OF  MINERALS  43 

2.  Alcohol  added  to  a  solution  of  a  borate  will  burn  with  a 
green  flame. 

3.  Turmeric  paper  moistened  with  a  HC1  solution  of  a  borate, 
and  dried  carefully  on  the  outside  of  the  test-tube  containing  the 
boiling  solution,  becomes  reddish-brown.     This  color  is  changed  to 
black  by  NH4OH.     It  is  well  to  run  a  blank  test  at  the  same  time. 
Zirconium  solutions  give  a  similar  test. 

Calcium,  Ca. 

1.  In  a  rather  concentrated  solution,  dilute  H2S04  precipitates 
CaS04-2H2O,    which   appears    crystalline 

with  the  hand  lens  in  contrast  with  BaSO4 
and  SrSO4.  The  addition  of  50  per  cent, 
ethyl  alcohol  makes  a  very  complete 
precipitation. 

2.  The  microchemical   gypsum  test  is 
the   most   satisfactory  test  for  calcium. 
A  drop  of  solution  containing  calcium  is 

placed  on  a  glass  slip  and  alongside  of  it     FIG.  4.— Microchemical 
a  drop  of  dilute  H2SO4.     The  two  drops 

are  brought  into  contact.     In  a  few  minutes  time  small  crystals 
of  CaSO4-2H2O  (gypsum)  make  their  appearance.     (See  Fig.  4.) 

3.  Yellowish-red  flame  with  HC1. 

4.  (NH4)2C2O4  or  (NH4)2CO3  gives  a  white  precipitate  soluble 
in  acids,  as  do  also  Ba  and  Sr.     (Ba  gives  a  yellow  precipitate 
with  K2CrO4  in  the  presence  of  dilute  acetic  acid.     Ca(NO3)2 
is  soluble  in  ether-alcohol,  while  Sr(NO3)2  is  insoluble.) 

5.  Calcium  borates,  fluorids,  and  phosphates  are  all  precipi- 
tated from  acid  solution  on  the  addition  of  NEUOH,  and  hence 
may  be  confused  with  aluminum  hydroxid.     In  this  case  the 
calcium  may  be  detected  as  given  in  Note  1  above. 

Carbon,  C. 

A.  Carbonates 

1.  Carbonates  effervesce  in  dilute  acids  (some  in  the    cold, 
others  only  upon  heating)  with  the  evolution  of  a  colorless,  odor- 


44          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

less  gas  which  gives  a  white  precipitate  with  Ba(OH)2  or  lime- 
water. 

2.  Carbonates  effervesce  in  a  hot  borax  bead.     When  the  bead 
cools,  a  mass  of  tiny  bubbles  may  be  detected  with  a  lens. 

3.  Citric  acid,  a  solid,  serves  as  a  convenient  field  reagent. 
Carbonates  effervesce  in  a  water  solution  of  citric  acid. 

B.  Hydrocarbons 

1.  Hydrocarbons,  such  as  asphaltum,  albertite,  bituminous 
coal,  etc.,  which  are  mineraloids  rather  than  true  minerals, 
when  heated  in  the  closed  tube  give  oils  and  tar-like  substances 
with  a  characteristic  disagreeable  odor. 

Chlorin,  Cl. 

1.  To  a  NaPO3  bead  saturated  with  CuO  (or  malachite)  a  little 
of  the  powdered  substance  is  added.     On  heating,  an  intense 
azure-blue  flame  is  obtained. 

2.  Insoluble  chlorids  fused  first  with  Na2C03,  and  then  heated 
with  MnO2  and  an  excess  of  KHS04  in  a  closed  tube,  give  free 
chlorin. 

3.  In  chlorid  solutions  AgNO3  gives  a  white  curdy  precipitate 
which  is  soluble  in  NH^OH. 

Chromium,  Cr. 

1.  The  borax  and  sodium  metaphosphate  beads  are  emerald 
green  in  both  O.F.  and  R.F. 

2.  The  sodium  carbonate  bead  is  yellow  in  O.F.     A  little 
KNO3  or  NaN03  helps  the  reaction. 

3.  Chromate  solutions  give  a  dark  red  precipitate  with  AgNO3 
and  a  yellow  precipitate  with  Pb(C2H302)2. 

Cobalt,  Co. 

1.  The   borax  and   sodium  metaphosphate   beads  are  deep 
blue  in  both  O.F.  and  R.F.     This  furnishes  a  very  delicate  test 
for  cobalt. 

2.  Heated   on   charcoal  in  R.F.,  cobalt  compounds  become 
magnetic  as  do  also  nickel  and  iron  compounds. 


CHEMICAL  PROPERTIES  OF  MINERALS  45 

Copper,  Cu. 

1.  Green  flame  made  azure-blue  with  HC1. 

2.  Borax  and  sodium  metaphosphate  beads  are  blue  in  O.F., 
and  opaque  red  (due  to  Cu2O)  in  R.F.  if  large  amounts  are  used. 
Metallic  copper  may  also  be  formed  in  R.F.     In  the  presence  of 
iron,  the  O.F.  bead  is  green  or  bluish-green. 

3.  On  charcoal  with  Na2CO3  in  R.F.,  and  also  with  NaP03 
and   metallic   tin   on   charcoal,   metallic   copper   (malleable)   is 
obtained. 

4.  Solutions  of  copper  minerals  are  blue  (green  in  the  presence 
of  iron).     NH4OH  in  excess  produces  a  deep  blue  coloration. 
(Nickel  solutions  give  a  faint  blue  coloration  with  NH^OH.) 

5.  A  slightly  acid  copper  solution  touched  to  a  bright  surface 
of  iron,  such  as  knife-blade  or  hammer,  gives  a  coating  of  metallic 
copper. 

Fluorin,  F. 

1.  Fluorids  are  soluble  in  concentrated  H2SC>4  with  evolution 
of  HF  which  etches  glass.     A  lead  dish,  or  watch-glass  coated 
with  paraffin,  should  be  used. 

2.  Fluorin  compounds  heated  in  a  closed  tube  with  4  parts  of 
NaPO3  will  etch  glass,  and  deposit  a  ring  of  SiO2  which  cannot  be 
washed  off  with  water. 

3.  Fluorin  compounds  heated  with  concentrated  H2S04  and 
powdered  silica  give  fumes  which  condense  on  moistened  black 
paper. 

4.  Fluorids  give  a  momentary  green  flame  when  heated  with 
borax  and  KHSO4.     This  flame  is  due  to  the  formation  of  vola- 
tile BF3. 

Gold,  Au. 

1.  With  sodium  carbonate  on  charcoal,  gold  compounds  give  a 
malleable  yellow  button. 

2.  Gold  may  be  identified  in  some  of  its  rich  ores  by  panning 
and  washing  away  light  quartz,  rock,  etc.     Mercury  is  added  to 


46          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

the  concentrates.  By  grinding  in  a  mortar,  an  amalgam  of  gold 
is  obtained.  This  may  be  heated  on  charcoal  or  in  a  closed 
tube,  and  the  mercury  driven  off.  When  the  residue  is  heated 
with  a  little  borax  on  charcoal  a  globule  of  gold  is  obtained. 

Hydrogen,  H. 

1.  Minerals  with  so-called  water  of  crystallization  give  water 
when  heated  in  a  closed  tube  at  a  comparatively  low  temperature 
(100-150°  C.).     With  hydrous  sulphates  of  iron,  copper,    and 
aluminum  the  water  has  an  acid  reaction  which  is  due  to  the 
SO   8;iven  off.      / 

2.  /:     1  salts  and  basic  salts  give  water  at  comparatively 
high  temperatures  (usually  above  150°  C.). 

Iron,  Fe. 

1.  On  charcoal  in  R.F.,  especially  with  sodium  carbonate, 
iron  minerals  become  magnetic.     (This  test  must  be  tried  after 
the  assay  has  become  cold.)     Cobalt  and  nickel  compounds  give 
a  similar  test. 

2.  In  O.F.  the  borax  bead  is  amber  colored,  and  in  R.F.,  pale 
green. 

3.  NH4OH  precipitates  brownish-red  Fe(OH)3  from  solutions 
containing  ferric  iron.     A  few  drops  of  HNO3  should  always  be 
added  to  the  solution  to  insure  oxidation  of  the  iron  to  the  ferric 
condition. 

4.  To  detect  state  of  the  iron,  a  borax  bead  made  blue  with 
CuO  (or  malachite)  is  changed  to  opaque  red  by  a  ferrous  com- 
pound,  and  to  green  by  a  ferric  compound.     (Use  a  neutral 
flame.) 

5.  To  detect  the  state  of  iron  in  insoluble  minerals  (especially 
silicates),  fuse  powdered  mineral  with  a  large  excess  of  borax  in  a 
test-tube.     Break  the  tube  and  dissolve  finely  powdered  contents 
in  HC1.     Divide  the  solution  in  two  portions  and  test  one  with 
K4Fe(CN)6   (ferric   compounds   give   a  deep   blue   precipitate) 
and  the  other  with  K3Fe(CN)6  (ferrous  compounds  give  a  dark 
blue  precipitate). 


CHEMICAL  PROPERTIES  OF  MINERALS  47 

Lead,  Pb. 

1.  On  charcoal  with  sodium  carbonate  in  R.F.  a  malleable 
button  of  lead  and  a  yellow  coating  of  PbO.     PbS  also  gives  a 
white  coating  of  PbSO4 

2.  On  plaster  with  iodid  flux,  lead  compounds  give  a  lemon- 
yellow  coating. 

3.  From  nitric  acid  solutions  containing  lead,  HC1  precipitates 
PbCl2,  which  is  soluble  in  the  hot  solution,  but  recrystallizes  on 
cooling  the  solution  as  white  acicular  crystals  with  adamantine 
luster. 

Lithium,  Li. 

1.  A  purplish-red  flame,  most  intense  at  first. 

2.  For  separation  from  the  other  alkalies,  see  item  34,  page  40. 

Magnesium,  Mg. 

1.  In  the  presence  of  NH4OH  and  NH4C1,  Na2HPO4  precipi- 
tates  NH4MgP04'6H20,    which   forms    slowly.     The    solution 
should  be  cold.     Other  metals  (except  alkalies)  must  be  absent 
as  they  also  give  precipitates. 

2.  White   magnesium   compounds   give   a   pink   color   when 
ignited  with  cobalt  nitrate  solution.     (This  test  is  not  very 
satisfactory). 

Manganese,  Mn. 

1.  The  sodium  carbonate  bead  is  bluish-green  and  opaque 
(a  very  delicate  test). 

2.  The  borax  or  NaPO3  bead  is  amethyst  colored  in  O.F.  and 
colorless  in  R.F.     Large  amounts  of  iron  interfere  with  this  test. 

3.  With  HC1  manganese  dioxids  give  off  chlorin,  a  gas  recog- 
nized by  its  penetrating  odor. 

Mercury,  Hg. 

1.  In  closed  tube  with  dry  sodium  carbonate,  mercury  com- 
pounds give  metallic  globules  of  mercury. 

2.  On  plaster  with  iodid  flux  a  scarlet  sublimate  when  gently 
heated.     If  overheated,  the  sublimate  is  dark  greenish-yellow. 


48  INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

3.  Most  mercury  compounds  rubbed  on  a  copper  coin  with 
HC1  give  a  white  amalgam. 

Molybdenum,  Mo. 

1.  The  NaP03  bead  is  green  in  R.F.,  but  colorless  in  O.F. 
Several  R.F.  beads  dissolved  in  HC1  with  tin  give  a  brown 
solution. 

2.  Na2HP04  gives  a  yellow  precipitate  with  hot  nitric  acid 
solutions  of  molybdenum  compounds. 

Nickel,  Ni. 

1.  The  borax  bead  in  O.F.  is  violet  when  hot,  reddish-brown, 
when  cold;  while  in  R.F.  the  bead  is  turbid  gray. 

2.  With  nickel  solutions  NaOH  gives  a  pale  green  precipitate 
which  is  insoluble  in  excess.     With  NH4OH  a  precipitate  is 
formed  which  is  soluble  in  excess  to  a  pale  blue  solution  (fainter 
than  copper). 

3.  A  one  per  cent,  solution  of  dimethyl-glyoxime  in  alcohol 
added  to  an  alkaline  solution  containing  nickel,  will  give  a  red 
precipitate. 

Niobium,  Nb. 

1.  When  fused  with  borax  and  then  dissolved  in  HC1,  the 
addition  of  metallic  tin  gives  a  deep  blue  solution  similar  to  that 
obtained  for  tungsten;  but,  unlike  the  latter,  the  color  disappears 
on  the  addition  of  water. 

Nitrogen,  N. 

1.  In  the  closed  tube  with  KHSO4,  nitrates  give  brown-red 
fumes  of  N02- 

2.  A  concentrated  solution  of  FeS04  added  to  a  solution  of  a 
nitrate  in  concentrated  H2S04  gives  a  brown  ring. 

Oxygen,  O. 

No  direct  tests  for  oxygen  are  easily  made.  The  dioxids  of 
manganese  dissolve  in  HC1  with  the  evolution  of  chlorin,  which  is 
recognized  by  its  odor  and  by  its  bleaching  effect  on  litmus  paper. 


CHEMICAL  PROPERTIES  OF  MINERALS  49 

Phosphorus,  P. 

1.  An  excess  of  (NEU^MoC^  added  to  a  hot  nitric  acid  solution 
of  a  phosphate  gives  a  yellow  precipitate  which  is  soluble  in 
NH4OH.     The   solution   should   be   only    slightly    heated,   for 
arsenates  give  a  similar  precipitate  on  boiling. 

2.  Most  phosphates  give  a  bluish-green  flame  when  moistened 
with  H2S04. 

Platinum,  Pt. 

1.  Metallic  platinum  is  insoluble  in  any  single  acid,  but 
soluble  in  aqua  regia.  In  rather  concentrated,  slightly  acid 
solutions,  KC1  gives  a  yellow  precipitate,  K2PtCl6,  insoluble  in 
alcohol. 

Potassium,  K. 

1.  Violet  flame,  masked  by  sodium,  but  visible  through  a  blue 
glass.     (Merwin's  flame-color  screen  is  better  than  a  blue  glass.) 
Potassium  in  silicates  may  be  detected  by  fusing  with  Na2C03 
and  observing  the  flame  through  a  blue  glass  or  screen. 

2.  Sodium  cobaltic  nitrite,  Na3Co(N02)6,  (see  p.  25),  gives  a 
yellow  precipitate  insoluble  in  alcohol. 

3.  With  H2PtCl6,  potassium  solutions  give  a  yellow  crystalline 
precipitate  (K2Ptde)  insoluble  in  95  per  cent,  alcohol. 

Silicon,  Si. 

1.  In  the  NaP03  bead,  silica  and  the  silicates  are  partially 
dissolved  and  usually  leave  a  translucent  mass  or  skeleton  of 
SiO2.     (A  few  other  minerals  such  as  corundum  are  soluble  with 
difficulty.) 

2.  With  a  small  amount  of  sodium  carbonate,  silica  effervesces 
and  forms  a  clear  mass.     The  equation  is:  Na2CO3  +  Si02  = 
Na2SiO3  +  C02. 

3.  Some  silicates  dissolve  in  HN03  or  HC1,  and  on  evaporation 
leave  either  a  gelatinous  mass  or  a  slime  of  silicic  acid. 

4.  For  insoluble  silicates  a  sodium  carbonate  fusion  must  be 
made.     The  finely  powdered  mineral  is  fused  on  platinum  foil, 


50          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

or  a  spiral  loop  of  platinum  wire,  with  three  to  four  parts  of 
sodium  carbonate.  The  fused  mass  is  dissolved  in  dilute  HN03 
and  carefully  evaporated  just  to  dry  ness.  After  adding  dilute 
HC1  and  boiling,  the  insoluble  silica  is  filtered  off.  The  filtrate 
contains  the  metals,  which  are  commonly  Al,  Fe,  Ca,  and  Mg. 
The  following  is  a  scheme  of  separation: 

Add  NH4OH  and  NH4C1  Fe(OH)3,Al(OH;3 :  A1(OH)3  is  soluble  in  KOH. 
(boiling) 


Add  (NH4)2C2(X 
(hot) 


CaC204. 


(cold) 


Add  Na2HP04  MgNH4P04.6H20. 


5.  For  the  detection  of  alkalies  in  silicates,  the  very  finely 
powdered  substance  is  intimately  mixed  with  five  parts  of  CaCO3 
and  one  part  of  NH^Cl  and  fused  for  some  time  on  platinum  foil. 
The  sintered  mass  is  digested  in  hot  water  and  filtered.  NH4OH 
and  (NH4)2CO3  are  added  to  the  filtrate.  The  precipitate  is 
filtered  off  and  the  filtrate  evaporated  to  dryness.  The  residue 
is  ignited  until  all  the  ammonium  salts  are  volatilized,  and  then 
the  residue  is  dissolved  in  a  little  water.  On  the  addition  of 
H2PtCl6  and  95  per  cent,  alcohol,  a  yellow  precipitate  indicates  K. 
The  filtrate  is  evaporated  to  dryness  and  the  flame  tested  for  Na. 

Silver,  Ag. 

1.  With  soda  on  charcoal  in  R.F.,  silver  minerals  yield  malle- 
able metallic  globules  of  silver,  which  may  be  tested  as  under  3. 

2.  For  the  blowpipe  silver  assay  see  page  34. 

3.  On   the  addition   of  HC1  nitric   acid   solutions  of  silver 
minerals  give  a  white  curdy  precipitate  which  changes  to  violet 
on  exposure  to  light  and  is  soluble  in  NH4OH. 

Sodium,  Na. 

1.  Intense  yellow  flame  masked  by  a  thick  blue  glass  (or 
Merwin  flame-color  screen) .  This  is  such  a  delicate  test  that  only 
an  intense  and  prolonged  coloration  indicates  sodium  as  an 
essential  constituent, 


CHEMICAL  PROPERTIES  OF  MINERALS  51 

2.  Sodium  in  insoluble  silicates  may  be  detected  by  the  method 
given  under  Silicon,  note  5. 

Strontium,  Sr. 

1.  Strontium   compounds   give   a   crimson   flame,   especially 
with  HC1. 

2.  Dilute  H2SO4  gives  a  white  precipitate,  SrSO-i,  with  dilute 
strontium  solutions.     Barium  solutions  give  the  same  test,  and 
may  be  distinguished  by  the  fact  that  K2CrO4  precipitates 
BaCrO4  from  an  acetic  acid  solution  while  the  strontium  remains 
in  solution. 

Sulfur,  S. 

A .  Sulfids  and  Sulfo-salts. 

1.  The  finely  powdered  substance  fused  with  three  parts  of 
sodium  carbonate  on  a  sheet  of  mica  (or  platinum  foil)  gives  a 
mass    which    stains    a   moistened   silver   coin.     Tellurids   and 
selenids  give  the  same  test. 

2.  In  the  closed  tube  some  sulfids  (e.g.  pyrite)  give  a  yellow 
sublimate  of  sulfur. 

3.  In  the  open  tube  sulfids  give  off  S02,  a  colorless  gas  with  the 
odor  of  burning  matches. 

4.  A  few  sulfids  (e.g.,  sphalerite)  dissolve  in  HC1  with  the 
evolution  of  H2S. 

5.  Sulfids  are  oxidized  to  sulfates  by  nitric  acid  with  the  evolu- 
tion of  brown-red  fumes  of  N02-     The  solution  may  be  tested 
as  under  7  below. 

B.  Sulfates. 

6.  A  sulfate,  powdered  and  thoroughly  mixed  with  3  parts  of 
soda  and  a  little  charcoal  powder,  is  fused  on  charcoal  in  R.F. 
The  fused  mass  will  stain  a  moistened  silver  coin.     Sulfids  give 
the  same  test,  so  it  is  necessary  to  try  the  test  on  mica  or  platinum 
first. 

7.  Sulfate  solutions  with  BaCl2,  give  a  white  precipitate  which 
is  insoluble  in  HC1.     If  the  mineral  is  insoluble  in  acids  it  must 


52          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

be  fused   with  NaoC03,   and  the  water  solution  of  the  fusion 
used. 

8.  Sulfate  solutions  will  give  the  microchemical  gypsum  test 
with  a  calcium  salt  (calcite  dissolved  in  HC1  is  convenient). 
See  Calcium,  note  2,  page  43. 

Tellurium,  Te. 

1.  A  powdered   tellurid   added  to  hot   concentrated   H2S04 
gives  a  fine  red-violet  coloration. 

2.  Tellurids  give  a  pale  bluish-green  flame  coloration  and  a 
white  sublimate  on  charcoal. 

3.  On  plaster  with  bismuth  flux,  a  purplish  sublimate  is  ob- 
tained with  tellurids.     (Like  that  for  bismuth  except  that  there 
is  no  underlying  scarlet.) 

Tin,  Sn. 

1.  Cassiterite,  wrapped  in  zinc  shavings  or  placed  on  a  sheet  of 
zinc,  and  treated  with  dilute  HC1  becomes  coated  with  metallic  tin. 

2.  Tin  compounds  heated  on  charcoal  in  O.F.  give  a  straw- 
colored  coating,  Sn02.     On  addition  of  Co  (NO  3)2  solution  and 
heating  in  R.F.,  a  bluish-green  coloration  results. 

3.  Tin  compounds  fused  on  charcoal  with  sodium  carbonate 
and  a  little  sulfur  in  strong  R.F.  give  malleable  metallic  buttons 
of  tin  which  are  oxidized  by  HN03  to  a  white  insoluble  powder, 
H2Sn03. 

Titanium,  Ti. 

1.  The   NaP03   bead   saturated   with   the   finely   powdered 
mineral  is  violet  in  R.F.  and  colorless  in- O.F. 

2.  Fused  with  Na2CO3;  dissolved  in  HC1,  and  the  solution 
heated   with  metallic  tin,   titanium   compounds  give  a  violet- 
colored  solution  due  to  the  formation  of  TiCl3.     The  solution  is 
usually  turbid  due  to  the  formation  of  metatitanic  acid,  H2TiO3. 

3.  A  yellow  coloration  results  when  a  solution  of  hydrogen 
peroxide  is  added  to  the  substance  fused  with  KHS04.     This  is 
a  very  delicate  test. 


CHEMICAL  PROPERTIES  OF  MINERALS  53 

Tungsten,  W. 

1.  The  NaPO3  bead  is  blue  in  R.F.,  colorless  in  O.F.     Iron 
interferes  and  gives  a  red  bead  in  R.F. 

2.  NaP03  beads  treated  on  charcoal  in  R.F.  with  tin  are  dis- 
solved in  HC1  and  on  the  addition  of  metallic  tin,  a  deep  blue 
solution   results.     Niobates   give   a  similar  test  but  the  blue 
coloration  disappears  on  the  addition  of  water  in  this  case. 

3.  With  soluble  tungstates  HC1  gives  a  yellow  residue,  WO3, 
which  is  soluble  in  NH4OH.     On  the  addition  of  tin  and  boiling, 
the  precipitate  becomes  blue. 

Uranium,  U. 

1.  The  NaP03  bead  is  a  fine  green  in  R.F.  and  yellowish-green 
in  O.F. 

Vanadium,  V. 

1.  The  NaPO3  bead  is  a  fine  green  in  R.F.;  light  yellow  in  O.F. 

2.  In  the  closed  tube  with  KHSOi,  vanadates  give  a  yellow 
mass. 

Water  (see  Hydrogen). 
Zinc,  Zn. 

1.  On  charcoal  with  sodium  carbonate,  zinc  compounds  give  a 
white  coating  which  is  yellow  when  hot.     With  silicates  the 
addition  of  borax  helps. 

2.  Zinc  minerals,  when  moistened  with  Co  (NO  3)2  solution  and 
intensely  ignited,  assume  a  bright  green  color  which  is  due  to  the 
formation  of  cobalt  zincate.     Zinc  silicates  give  a  blue  color  like 
aluminum  compounds,  but  if  tried  on  charcoal  the  sublimate  will 
turn  green. 

3.  (NH4)2S  precipitates  ZnS  in  alkaline  solutions  which  is 
remarkable  as  being  the  only  insoluble  white  sulfid. 

Zirconium,  Zr. 

1.  An  HC1  solution  of  a  soda  fusion  turns  turmeric  paper  orange 
color.  This  test  is  like  that  for  a  borate,  the  absence  of  which 
must  be  proved. 


THE  MORPHOLOGICAL  PROPERTIES  OF  MINERALS 

Minerals  may  occur  in  two  essentially  different  conditions  or 
states:  (1)  the  crystalline  and  (2)  the  amorphous.  In  a  crystal- 
line substance  many  of  the  physical  properties  such  as  cleavage 
and  hardness,  for  example,  vary  with  the  direction,  while  on  the 
other  hand,  in  an  amorphous  substance  the  physical  properties 
are  the  same  in  all  directions.  Of  the  various  directional  or 
vectorial  properties,  some  vary  continuously  and  can  be  repre- 
sented by  a  curve,  while  the  others  have  sharp  breaks  and  so  are 
called  discontinuous.  (See  Figs.  299  and  300  on  page  148.) 

A  crystalline  substance  is  a  homogeneous  substance  with  discon- 
tinuous vectorial  properties  (Friedel.)  Cleavage  is  one  of  the 
prominent  discontinuous  vectorial  properties.  Crystalline  sub- 
stances when  formed  under  favorable  conditions  in  a  free  space 
usually  take  on  a  geometric  form  characteristic  of  the  substance. 
Such  crystals  are  said  to  be  euhedral.  Rock  crystal  (quartz) 
furnishes  a  good  example.  An  irregular  fragment  of  quartz 
is  still  a  crystal,  for  it  has  exactly  the  same  physical  and  chemical 
properties  as  the  perfect  geometric  form.  A  crystal  without 
external  faces  is  said  to  be  anhedral,  while  crystals  with  imper- 
fectly developed  faces  are  said  to  be  subhedral.  These  terms 
are  necessary  in  order  to  avoid  the  ambiguity  in  the  use  of  the 
term  crystal. 

1.    THE  AMORPHOUS  CONDITION 

Amorphous  substances  in  free  spaces  assume  a  more  or  less 
spherical  form.  In  this  respect  they  resemble  liquids,  for  the 
shape  of  a  liquid  free  from  external  influences  is  spherical.  For 
example,  olive  oil  in  a  mixture  of  alcohol  and  water  of  exactly 
the  same  specific  gravity  takes  on  the  form  of  spheres.  A  variety 
of  opal  from  Tateyama,  Japan,  occurs  in  small  spheres.  But  in 
most  cases  amorphous  minerals  formed  in  free  spaces  are  in- 

54 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS  55 

fluenced  by  gravity  and  for  this  reason  they  appear  in  mam- 
millary  (hemispherical  protuberances),  botryoidal  (more  or  less 
separated  spheres  like  a  bunch  of  grapes),  and  stalactitic  (pen- 
dant like  icicles)  forms.  Colloform  is  a  general  term  coined  by 
the  author  to  cover  all  these  forms.  A  colloform  structure  is 
also  assumed  by  microcrystalline  minerals  such  as  chalcedony. 
The  forms  assumed  by  all  amorphous  minerals  are  practically 
the  same,  while  on  the  other  hand  the  crystals  of  each  mineral 
are  characteristic  of  that  mineral.  The  amorphous  minerals  are 
hardened  hydrogels;  all  are  probably  colloidal  in  origin-  (See 
~P-  17.) 

2.  THE  GENERAL  PROPERTIES  OF  CRYSTALS 

In  beginning  the  study  of  crystals,  the  student's  attention 
may  be  directed  to  crystals  of  the  common  minerals  such  as 
calcite  (Figs.  158-169),  quartz  (Figs.  180-183),  pyrite  (Figs.  113- 
120),  gypsum  (Figs.  224-227),  and  orthoclase  (Figs.  212-215). 
Then,  for  the  time  neglecting  how  they  were  formed  and  what 
they  are  composed  of,  their  form  or  geometrical  properties  may 
be  considered. 

Euhedral  crystals  are  naturally  formed  solids  bounded  by 
flat,  more  or  less  smooth  surfaces  called  faces,  which  are  the 
result  of  an  internal  structure.  (The  surfaces  on  cut  gems  are 
known  as  facets.)  Intersections  of  two  faces  are  called  edges, 
and  intersections  of  three  or  more  faces  are  called  vertices. 
The  faces  of  crystals  vary  greatly  in  number,  in  shape,  and  in 
position.  On  many  crystals  it  will  be  noticed  that  there  are 
several  kinds  of  faces.  All  the  faces  of  one  kind  on  a  crystal 
constitute  a  form.  For  example,  in  Fig.  13  the  top  and  bottom 
six-sided  faces  constitute  one  form,  and  the  six  rectangular 
faces  another  form.  Some  crystals  have  only  a  single  form,  but 
most  of  them  are  combinations  of  two  or  more  forms.  It  is  the 
great  number  of  combinations  possible  that  gives  the  variety  to 
crystals,  for  there  is  practically  no  limit  to  the  number  of  forms 
possible  on  a  crystal. 


56          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

The  arrangement  of  crystal  faces  in  belts  of  planes  with 
parallel  intersection  edges  called  zones  is  a  notable  feature  of 
most  crystals.  For  example,  the  six  faces  of  Fig.  37  constitute 
a  zone. 

One  of  the  most  striking  and  important  properties  of  crystals  is 
the  recurrence  of  the  faces,  edges,  and  vertices  according  to 
some  fixed  law.  This  property  is  known  as  symmetry.  It 
varies  for  different  kinds  of  crystals,  and  is  the  basis  for  the  classi- 
fication of  crystals. 

On  looking  over  a  number  of  crystals  one  might  fail  to  see  any 
order,  system,  or  regularity  so  great  is  their  variety,  and  might 
decide  that  crystals  are  fortuitous  solids.  But  such  is  not  the 
case;  for  between  the  faces,  angles,  and  zones  of  crystals  there 
exist  exact  mathematical  relations.  Given  the  angles  between 
a  few  faces  of  a  crystal,  the  angles  between  any  two  of  the  many 
crystal  faces  possible  may  be  calculated.  Crystal  faces  intersect 
only  at  certain  definite  angles.  A  facet  cut  at  random  on  a  crys- 
tal is  not  a  crystal  face,  for  the  faces  are  the  result  of  a  definite 
internal  structure. 

The  practical  importance  of  crystallography  lies  in  the  fact 
that  a  given  mineral  or  artifically  prepared  compound  often 
occurs  in  crystals  characteristic  of  that  substance,  and  hence  the 
crystal  form  may  be  used  in  the  determination  of  the  substance. 

3.  THE  MEASUREMENT  OF  CRYSTALS 

The  angles  on  any  crystal  are  the  plane  angles  of  the  faces,  the 
interfacial  or  dihedral  angles  over  the  edges,  and  the  solid  or 
polyhedral  angles  at  the  vertices.  On  account  of  the  difficulty  of 
accurate  measurement,  the  plane  angles,  though  characteristic,  are 
little  used.  The  measurement  of  interfacial  angles  is  the  start- 
ing-point in  the  description  and  determination  of  crystals.  An 
interfacial  angle  (dihedral  angle  of  geometry)  is  defined  by  the 
plane  angle  that  is  formed  by  cutting  a  plane  normal  to  the  inter- 
section edge  of  the  two  faces.  It  will  be  noticed  that  there  are 
two  possible  angles  to  measure:  an  internal  and  an  external  or 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


57 


supplement  angle.  For  various  reasons1  the  supplement  angle  is 
the  one  used  in  crystallography.  In  a  hexagonal  prism,  for 
example,  the  interfacial  angle  is  read  60°  instead  of  120°.  The 
interfacial  angle  may  be  measured  approximately  by  means  of 
a  contact  goniometer,  which,  in  the  simplest  type,  consists  of  a 
semicircular  cardboard  protractor  provided  with  a  celluloid  arm 
(Fig.  5) .  The  plane  of  the  protractor  is  placed  perpendicular  to 


FIG.  5. —  Contact  goniometer. 

the  intersection  edge.  One  face  of  the  crystal  is  brought  in 
contact  with  the  arm  and  the  protractor  is  revolved  until  the 
other  face  is  parallel  to,  but  not  quite  in  contact  with,  it. 

For  more  accurate  work,  especially  on  minute  crystals,  the 
reflection  goniometer  is  used.  The  principle  of  measurement  is 
as  follows:  if  a  bright  face  of  a  crystal  is  held  close  to  the  eye,  a 


1  U)  The  sum  of  the  supplement  angles  in  any  zone  is  equal  to  360°. 
obtained  directly  from  the  reflection  goniometer  are  the  supplement  angles, 
to  estimate  the  supplement  angle  with  the  eye. 


(2)  The  angles 
(3)  It  is  easier 


58 


INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


reflection  of  a  distant  object  such  as  a  window  bar  may  be 
obtained.  If  the  crystal  is  turned  about,  reflections  are  obtained 
from  other  faces.  The  angle  through  which  the  crystal  is 
revolved  to  obtain  the  images  from  two  adjoining  faces  is  the 
external  or  supplement  angle. 

The  reflection  goniometer,  the  invention  of  Wollaston  in  1809, 
originally  consisted  of  a  vertical  graduated  circle  with  a  horizon- 


FIG.  6. — Reflection  goniometer. 

tal  axis  bearing  the  crystal  carrier.  In  the  modern  type  of  goni- 
ometer the  graduated  circle  is  horizontal  and  the  axis  of  revolu- 
tion is  vertical.  Fig.  6  shows  a  convenient  type  of  goniometer 
for  student  work.  A  central  axis  s  bears  a  crystal  carrier  (axis 
of  the  graduated  circle)  with  adjustments  which  are  two  sliding 
motions  (q  and  r)  at  right  angles,  and  two  tipping  motions  on 
circular  arcs  at  right  angles  (o  and  n).  A  collimator  A  with  a 
biconcave  slit  at  the  end,  and  a  telescope  B  which  may  be  set  at 
any  angle  to  the  collimator,  complete  the  equipment.  A  source 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS  59 

of  light,  such  as  a  Welsbach  burner,  placed  at  the  end  of  the 
collimator  furnishes  a  beam  of  light  which  is  reflected  from  the 
crystal,  when  in  a  certain  position,  into  the  telescope.  Looking 
into  the  telescope  one  sees  a  biconcave-shaped  image  (Fig.  7) 
which  may  be  bisected  by  the  cross-wires  in  the  telescope. 
Then  the  reading  on  the  vernier  of  the  graduated  circle  is  taken. 
The  crystal  carrier  with  the  mounted  crystal  is  revolved  until 
an  image  is  reflected  from  another  face,  and  so  on  for  all  faces  of 
the  zone.  A  separate  set-up  must  be  made  for  each  zone. 

Another  type  of  goniometer  is  the  two-circle  or  theodolite 
goniometer  which  consists  of  two  graduated  circles  at  right 
angles.  Two  angles,  one  corresponding 
to  the  longitude  and  the  other  to  the  co- 
latitude  of  a  place  on  the  earth's  surface, 
are  obtained  for  each  face.  The  advan- 
tage of  the  two-circle  goniometer  lies  in 
the  fact  that  only  one  set-up  is  required 
for 'all  the  faces  on  one-half  of  a  crystal, 
but  the  disadvantage  is  that  in  monoclinic 
and  triclinic  crystals  measurements  are  FIG.  7. — Image  obtained 

,  j      .  with  reflection  goniometer. 

not  always  made  in  zones. 

In  the  case  of  small  crystals  with  dull  faces  the  polarizing 
microscope  with  rotating  stage  may  be  used  to  advantage  in 
measuring  angles  (see  Fig.  332,  page  174). 

A  simple  reflection  goniometer  may  be  made  from  the  card- 
board contact  goniometer  by  fitting  a  wooden  axis  through  the 
eyelet,  the  axis  being  provided  with  a  wire  pointer.  Fig.  8 
illustrates  this  device.  By  holding  the  goniometer,  with  the 
crystal  mounted  on  the  end  of  the  axis  with  wax,  so  that  the 
intersection  edge  of  the  faces  is  in  line  with  the  axis,  an  image 
of  a  distant  object,  such  as  a  window-bar,  on  a  crystal  face 
is  made  to  coincide  with  the  edge  of  a  table  or  similar  line  of 
reference.  The  reading  of  the  pointer  is  taken.  Then  after 
obtaining  the  same  image  again,  the  goniometer  is  held  firmly 
and  the  axis  carrying  the  crystal  is  rotated  until  a  similar  image 


60 


INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


is  obtained  from  an  adjacent  face.     The  supplement  angle  is  the 

difference  between  the  two  readings, 
and  so  on  for  other  faces  of  the 
zone.  As  the  protractor  includes 
but  180°,  only  part  of  the  zone  can 
be  measured  at  one  time. 

4.  THE  SYMMETRY  OF  CRYSTALS 

The  repetition  or  recurrence  of  the 
faces,  interfacial  angles,  and  vertices 
of  crystals  in  accordance  with  some 
fixed  law  is  called  symmetry. 
Symmetry  is  perhaps  the  most  im- 
portant property  of  crystals,  for 
among  natural  objects  it  is  a  pro- 
perty peculiar  to  crystals  (that  is,  if 
the  term  is  used  in  an  exact  mathe- 
matical sense)  and  besides  furnishes 
the  basis  for  the  classification  of 
crystals.  At  the  same  time  it  should 
be  emphasized  that  a  few  crystals 
lack  symmetry  of  any  kind  (e.g., 
hydrous  cal- 


FIG.  8. — Simple  reflection 
goniometer. 


cium   thiosul- 
fate,  CaS203-6H2O).     (See  Fig.  9.) 

The  symmetry  of  a  crystal  may  be  defined 
by  the  operations  necessary  to  bring  it  into 
coincidence  with  its  original  position.  The 
symmetry  operations  are  rotation  about 
an  axis,  reflection  in  a  plane,  a  combination 

»  ,.  .,,         „        .         /  FIG.  9. — A  crystal  (Ca- 

of  rotation  with  reflection  (rotatory-reflec-  s2o3-6H2O)     devoid    of 
tion),  and  inversion  about  the  center.         symmetry. 

If  a  solid  can  be  revolved  about  some  line  through  its  center  so 
that  similar  faces  recur  a  certain  number  of  times  in  a  complete 
revolution,  that  line  is  called  an  axis  of  symmetry  (denoted  by 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


61 


An).     In  crystals  the  period  of  the  axis  (the  value  of  n  in  the 
symbol  An)  or  the  number  of  times  of  recurrence  is  either  two 


FIG.  10.  FIG.  11.  FIG.  12. 

Figures  illustrating  axes  of  symmetry. 


FIG.  13. 


(2),  three  (3),  four  (4),  or  six  (6).  An  axis  about  which  similar 
faces  recur  every  180°  is  said  to  be  a  two- 
fold axis  (A2);  every  120°,  a  three-fold  axis 
(A3) ;  every  90°,  a  four -fold  axis  (A4) ;  and 
every  60°,  a  six-fold  axis  (A6).  Figs.  10, 
11,  12,  and  13  illustrate  these  various  axes 
of  symmetry.  The  vertical  lines  through 
the  centers  of  the  lower  figures  are  the  axes 
of  symmetry.  The  plane  figures  above  are 
plans  showing  the  amount  of  rotation  neces- 
sary to  bring  the  figures  into  self-coincidence. 
A  plane  that  divides  a  solid  into  two  parts 
so  that  similar  faces  occur  on  opposite  sides 
of  the  plane  is  called  a  plane  of  symmetry 
(denoted  by  P).  Unless  the  crystal  is  mis-shapen,  one  half  is 
the  mirror  image  of  the  other  half.  Fig.  14  represents  an  orthoclase 


FIG.     14.— Plane  of 
symmetry. 


62 


INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


crystal  in  which  the  shaded  area  is  a  plane  of  symmetry.  The 
faces  are  either  perpendicular  to  this  plane,  or  occur  in  pairs, 
one  on  each  side  of  it.  A  crystal  may  have  a  number  of  planes 
of  symmetry.  A  cube,  for  example,  has  nine,  three  parallel  to 
opposite  faces  and  six  through  opposite  edges. 

A  solid  is  said  to  have  a  center  of  symmetry  (denoted  by  C) 
if  a  line  drawn  from  any  point  through  the  center  encounters  an 
exactly  similar  point  on  the  opposite  side.  The  operation  is 
called  inversion.  Figure  15  represents  a  crystal  of  axinite  with  a 
center  of  symmetry.  Every  face  has  a  similar  parallel  face  on  the 
opposite  side  of  the  crystal.  This  is  the  easiest  test  for  a  center 
of  symmetry. 

The  recurrence  of  similar  faces  by  rota- 
tion about  an  axis,  combined  with  reflec- 
tion in  a  plane  normal  to  the  axis,  is  called 
composite  symmetry.  The  two  operations, 
rotation  and  reflection,  take  place  simul- 
taneously; therefore  the  symbol  &n  is  used. 
The  period  of  the  axis  is  always  even  and 
in  crystals  the  two  possible  cases  are  ^4 
and  ./Pe.  In  Fig.  16  the  vertical  line  is  an 
axis  of  4-fold  composite  symmetry,  for  the 
upper  part  of  the  crystal  revolved  90°  becomes  a  reflection  of 
the  lower  part.  Similarly  the  vertical  line  in  Fig.  17  is  an  axis  of 
6-fold  composite  symmetry,  for  the  crystal  revolved  60°  becomes 
a  reflection  of  the  lower  part.1  It  should  be  observed  that  ^ 
includes  A2,  and  ^6  includes  A3,  so  these  may  be  written  £>4  (A2) 
and  £>6  (A8). 

In  Fig.  15  the  front  part  of  the  crystal  when  revolved  180°  be- 
comes the  reflection  of  the  rear  part  (dotted  lines).  This  is  true, 
however,  of  any  direction  in  the  crystal,  so  that  &2  becomes  <» 
^2.  It  is  more  logical  to  use  C  than  °°  JP2,  for  a  single  opera- 
tion (inversion)  is  involved. 

1  A  twin-model  of  calcite  with  {OOOl}  as  twin-plane  may  be  used  to  show  composite  6- 
fold  symmetry. 


FIG.  15. — Center  of 
symmetry. 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS  63 

Center,  axis,  plane,  and  composite  axis  with  plane  are  collec- 
tively known  as  elements  of  symmetry.  In  crystals  the  ele- 
ments of  symmetry  are  combined  in  various  ways.  With  the 
limitation  imposed  by  the  rationality  of  indices,  or  with  the 
assumption  of  a  crystal  structure  made  up  of  particles  at  small, 
finite  distances  apart,  only  ax"es  of  2-,  3-,  4-,  and  6-fold  symmetry 
are  possible,  and  in  fact  no  other  axes  of  symmetry  have  ever  been 
found  in  crystals.  The  elements  of  symmetry,  then,  are  as 


FIG.  16.  FIG.  17. 

Figures  illustrating  composite  symmetry. 

follows:  A2,  A3,  A4,  AG,  P,  C,  -5*4,  and  JP&.  Various  methods  of 
combining  the  elements  of  symmetry  with  each  other  lead  to  the 
result  that  only  thirty-one  combinations  are  possible  among 
crystals.  These  thirty-one  combinations  of  symmetry  elements 
plus  the  crystal  division  without  any  symmetry  constitute  the 
thirty-two  crystal  classes.  L»* 4^  \* ,  so 

In  the  above  discussion  the  term  similar  faces  has  been  used  so 
often  that  an  explanation  is  necessary.  By  similar  faces  are 
meant  faces  which  are  more  or  less  alike  in  shape,  size,  and  ap- 
pearance. On  crystals  which  have  been  formed  quietly  without 


64          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

disturbing  influences,  similar  faces  have  the  same  size  and  shape. 
But  on  account  of  various  external  influences  similar  faces  are 
rarely  exactly  of  the  same  size  and  shape.  The  effect  of  external 
influences  may  be  illustrated  by  alum  which  crystallizes  in  octa- 
hedrons. Alum  crystallizing  on  the  bottom  of  a  beaker  will  form 
in  more  or  less  flattened  octahedrons  like  Fig.  18,  while  if  it 
crystallizes  about  a  weighted  string  suspended  in  the  solu- 
tion, the  crystals  will  be  more  or  less  perfect  octahedrons  like 
Fig.  19. 

The  irregularity  in  the  size  and  shape  of  similar  faces  is  one 
difficulty  in  the  study  of  crystals.     While  the  faces  may  vary,  the 


FIG.  18.  FIG.  19. 

Alum  crystals. 

angles  are  constant  (within  certain  limits),  as  is  expressed  in  the 
law  of  constancy  of  interf acial  angles :  In  all  crystals  of  the  same 
substance,  the  angles  between  corresponding  faces  are  constant. 
(Steno,  1669.)  In  order  to  determine  the  symmetry  of  a  crystal 
it  is  necessary  in  many  cases  to  measure  the  interfacial  angles. 
Thus  the  crystals  represented  in  cross-section  by  Figs.  20,  21,  22 
are  bounded  by  hexagonal  prisms  and  have  an  axis  of  six-fold 
symmetry  if  the  interfacial  angles  are  all  60°.  On  the  other 
hand,  the  crystal  represented  in  cross-section  by  Fig.  23,  though 
apparently  a  hexagonal  prism,  is  a  combination  of  two  forms 
(a  rhombic  prism  and  a  pinacoid),  and  has  an  axis  of  two-fold 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


65 


symmetry   and  not  one  of  six-fold  symmetry  because  the  inter- 
facial  angles  are  62°  and  56°  instead  of  60°. 

Another  property  of  crystals  used  in  determining  symmetry  is 
the  physical  character  of  the  faces;  and  for  this  reason  geomet- 
rical crystallography  is  by  no  means  merely  a  branch  of  ge- 
ometry. Similar  faces  are  those  with  the  same  luster,  the  same 


62 


60'       60  60 

FIG.  20.  FIG.  21. 


FIG.  22. 


kind  of  striations,  pits,  or  other  markings.  Geometrically  a 
cube  has  nine  planes  of  symmetry,  three  parallel  to  the  cube 
faces,  and  six  through  opposite  cube  edges,  but  a  cube  of  pyrite 
with  striations  like  those  of  Fig.  24  has  only  three  planes  (those 
parallel  to  the  cube  faces).  A  crystal  of  sphalerite  represented 


FIG.  24. 
Pyrite. 


FIG.  25. 
Sphalerite. 


FIG.  26. 
Apophyllite. 


by  Fig.  25  is  geometrically  an  octahedron,  but  from  the  stand- 
point of  crystallography  it  is  a  combination  of  two  tetrahedrons, 
one  with  smooth  faces,  the  other  with  striated  faces.  Figure  26 
illustrates  another  good  example  of  this  kind.  Apophyllite 
occurs  in  crystals  which  are  apparently  cubes  modified  by  the 
octahedron.  Close  examination,  however,  shows  that  the  side 


66 


INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


faces  are  striated  and  have  a  vitreous  luster,  while  the  top  and 
bottom  faces  are  smooth  and  have  a  pearly  luster.  The  forms, 
then,  are  a  pinacoid  and  square  prism  (tetragonal  prism)  instead 
of  a  cube.  The  apparent  octahedron  is  in  reality  a  double- 
ended  square  pyramid  (tetragonal  bipyramid  in  the  language 
of  crystallography.) 

A  more  general  method  of  determining  the  symmetry  of  a 
crystal  is  by  means  of  etch-figures.  When  a  crystal  is  acted 
upon  by  a  solvent,  the  action  is  not  uniform,  but  begins  at  certain 
points  and  proceeds  more  rapidly  in  some  directions  than  in 
others.  If  the  action  is  stopped  at  the  right  time,  the  faces  of 
the  crystal  are  usually  found  to  be  covered  with  little  angular 
_  figures  of  definite  shape  and  orientation  called 

etch-figures.  The  etch-figures  are  usually 
shallow  depressions  bounded  by  minute  faces, 
but  in  some  cases  they  are  elevations.  The 
fact  that  these  faces  are  often  general  forms 
(see  p.  72)  enables  one  in  many  cases  to 
determine  the  crystal  class.  Without  etch- 
ing it  would  have  been  impossible  to  assign 
many  crystalline  substances  to  their  proper 
crystal  class.  For  example,  the  representa- 
tives of  classes  9,  12,  23,  and  24  (see  p.  80,)  were  assigned  to 
these  classes,  solely  on  account  of  the  etch-figures  and  the  assign- 
ment of  many  crystals,  both  minerals  and  prepared  compounds, 
to  their  crystal  class  has  been  checked  by  the  etch-figures.  For 
example,  the  etch-figures  on  an  etched  prismatic  crystal  of 
nepheline  is  assigned  to  the  hexagonal  pyramidal  class  (A6) 
solely  on  account  of  the  etch-figures  (Fig.  27). 

The  shape  of  the  etch-figures  varies  with  the  solvent,  time, 
and  temperature,  but  whatever  their  shape  they  are  practically 
always  the  same  in  symmetry.  On  similar  faces  the  etch-figures 
are  alike  and  on  dissimilar  faces  they  are  unlike,  hence  we  have 
an  exact  method  of  determining  the  forms  present  on  the  crystal 
(see  Fig.  28).  The  faces  of  etch-figures  lie  in  well  developed 


Fon 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


67 


zones,  but  they  often  have  high  indices.  There  is  no  rule  to 
follow  in  obtaining  etch-figures  as  it  is  simply  a  question  of 
ease  of  solution.  Crystals  soluble  in  water  may  furnish  them  by 
passing  a  moistened  cloth  over  the  surface,  while  some  refractory 
minerals  such  as  topaz  require  fused  potassium  hydroxid. 

Crystals  are  sometimes  found  with  natural  etch-figures. 
Diamond  crystals,  for  example,  very  often  have  triangular  etch- 
figures  on  the  octahedral  faces  (see  Fig.  378,  p.  214). 

Closely  related  to  etch-figures  there  are  often  found  growth- 
figures  produced,  not  by  solution,  but  by  growth,  and  these  may 


FIG.  28. — Etch-figures  on  diopside. 
(Modified  from  Ries.) 


FIG.  29. — Growth-figures  on 
quartz  crystal. 


indicate  the  symmetry  of  the  crystal.  On  quartz,  for  example, 
these  are  sometimes  found  on  the  rhombohedral  f aces  r{  1011 J 
and  z{OlTl)  as  illustrated  in  Fig.  29. 

Optical  characters  are  also  useful  in  determining  the  true 
symmetry  of  a  crystal.  For  example,  stilbite  crystals  (see 
Fig.  574,  p.  409)  are  apparently  rhombic  bipyramidal,  but 
optical  examination  of  a  thin-section  parallel  to  the  cleavage 
face  b  proves  them  to  be  monoclinic  prismatic  crystals  twinned 
on  the  c  face,  i.e.,  they  are  composite  crystals  made  up  of  two 
individuals. 


68          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

6.  THE  FORMS  OF  CRYSTALS 

The  similar  faces  of  a  crystal  constitute  a  form,  the  word 
"form"  being  used  here  in  a  special  technical  sense.     Similarity 


FIG.  30. — Pedion.  FIG.  31. — Pinacoid.  FIG.  32. — Dome.  FIG.  33. — Sphenoid. 

of  faces  on  some  crystals  may  be  observed  at  a  glance,  but  for 
others  not  only  careful  examination  and  measurement,  but  also 


^~~~s—  v^       ~~^H^>i 

X 

- 

^ 

P^l 

H^ 

i 
i 

k 

b^-J 

i 
-j 

Is-.---... 

^—  "-i—  -  —  f  ~       —     •- 

FIG.  34.                      FIG.  35.                  FIG.  36.                      FIG.  37. 
Rhombic  prism.       Trigonal  prism.     Tetragonal  prism.     Hexagonal  prism. 

etching  with  some  solvent  is  necessary.  Similar  faces  will  have 
the  same  kind  of  etch-figures  as  has  been  mentioned  in  the  pre- 
ceding section. 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


There  are  many  kinds  of  forms.  The  most  logical  method  is 
to  name  the  forms  according  to  geometrical  principles,  regardless 
of  their  position  with  respect  to  axes  of  reference  (see  p.  73). 
A  single  face  is  called  a  pedion  (Fig.  30).  Two  parallel  faces 
constitute  a  pinacoid  (from  the  Greek  word  for  a  board)  (Fig.  31). 
A  form  composed  of  two  non-parallel  faces  is  known  as  a  dome 
(from  the  Latin  word  for  house)  if  astride  a  plane  of  symmetry 
(Fig.  32),  but  a  sphenoid1  (from  the  Greek  word  for  wedge), 
if  not  astride  a  plane  of  symmetry  (Fig.  33). 


Y.  ...—I 

..----,-1— 

FIG.  38.  FIG.  39.  FIG.  40. 

Ditrigonal  prism     Ditetragonal  prism.   Dihexagonal  prism. 

Next  we  have  three,  four,  six,  eight,  or  twelve  similar  faces  in 
one  zone.  These  are  called  prisms,  and  are  distinguished  accord- 
ing to  their  cross-section  as  trigonal  (Fig.  35),  rhombic  (Fig.  34), 
tetragonal  (Fig.  36),  hexagonal  (Fig.  37),  ditrigonal  (Fig.  38), 
ditetragonal  (Fig.  39),  and  dihexagonal  (Fig.  40).  Pyramids  are 
forms  consisting  of  three  or  more  similar  faces  intersecting  in  a 
point.  They  are  defined  by  the  shape  of  the  cross-section  just 
as  the  prisms  are.  See  Figs.  41  to  44,  and  49  to  51.  Bipyramids 

1  These  two  (dome  and  sphenoid)  are  known  by  different  names  because  one  results  from 
reflection  in  a  plane,  and  the  other  from  revolution  of  180°  about  an  axis. 


FIG.  45. 
Rhombic. 


FIG.  46. 
Trigonal. 


FIG.  47. 
Tetragonal. 


FIG.  48. 
Hexagonal. 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


71 


are  double-ended  forms  which  may  be  imagined  to  be  formed  by 
placing  two  pyramids  end  to  end.  They  are  defined  by  cross- 
section  just  as  prisms  and  pyramids  are.  See  Figs.  45  to  48 
and  52  to  54. 


FIG.  55.  FIG.  56.  FIG.  57. 

Trigonal  trapezohedron.  Tetragonal  trapezohedron.  Hexagonal  trapezohedron. 

Trapezohedrons  are  double-ended  forms  with  the  symmetry 
An-nA2.  They  are  distinguished  as  trigonal  (Fig.  55),  tetragonal 
(Fig.  56),  or  hexagonal  (Fig.  57),  according  to  the  period  n  of  the 
axis  An.  Bisphenoids  are  forms  consisting  apparently  of  two 
sphenoids  placed  together  symmetrically.  They  are  called 
rhombic  (Fig.  62),  or  tetragonal 
(Fig.  16),  according  to  cross-section. 
A  rhomb ohedron  is  a  form  consist- 
ing of  six  rhombic  faces,  three  at 
each  end  of  a  six-fold  axis  of  com- 
posite symmetry  (Fig.  17).  It  is 
like  a  cube  symmetrically  distorted 
along  one  of  its  diagonals.  Scale- 
nohedrons  are  double-ended  forms 
consisting  of  scalene  triangular  faces 
meeting  in  zigzag  lateral  edges. 
They  are  distinguished  by  their 
cross-section  as  tetragonal  (Fig.  58)  or  hexagonal  (Fig.  59). 

There  are  fifteen  more  kinds  of  forms  which  are  restricted  to 
the  isometric  system.  Some  of  these,  such  as  cube,  octahedron, 
and  tetrahedron  are  simple,  but  as  most  of  them  are  rather 


FIG.  58. 

Tetragonal 

scalenohedron. 


FIG.  59. 

Hexagonal 

scalenohedron. 


72          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

complicated,  their  description  is  deferred  until  the  isometric 
system  is  studied. 

Of  the  forty-eight  kinds  of  forms  possible,  all  but  four  have 
been  found  on  crystals.  The  four  are  the  tetragonal  trapezohe- 
dron,  the  ditetragonal  pyramid,  the  hexagonal  trapezohedron,  and 
the  dihexagonal  pyramid.  They  are  possible  forms  because  they 
each  have  the  symmetry  of  one  of  the  32  crystal  classes.  See 
table  on  p.  80. 

The  thirty-two  forms  which  result  from  the  symmetry  opera- 
tions performed  on  a  given  face  are  general  forms.  The  other 
sixteen  forms  result  when  the  face  occupies  a  special  position 


FIG.  60.  FIG.  61. 

Congruent  tetrahedra. 


FIG.  62.  FIG.  63. 

Enantiomorphous  rhombic  bisphenoids 


with  respect  to  the  elements  of  symmetry.  Thus  a  hexagonal 
prism  results  in  class  23  when  a  face  is  parallel  to  the  A6 . 

Of  the  above  mentioned  forms,  the  pyramids,  prisms,  pina- 
coid,  dome,  sphenoid,  and  pedion  cannot  occur  by  themselves; 
and  for  that  reason  are  called  open  forms.  All  the  others  are 
called  closed  forms  because  by  themselves  they  enclose  space. 

Two  forms  are  said  to  be  congruent  if  one  of  them  may  be 
made  coincident  with  the  other  by  rotation.  For  example,  the 
tetrahedra  of  Figs.  60  and  61  are  congruent.  Two  forms  are  said 
to  be  enantiomorphous  if  they  are  non-superposable  and  the 
mirror-image  of  each  other.  (The  right  hand  and  the  left  hand, 
for  example,  are  enantiomorphs) .  Thus  the  rhombic  bisphenoids 
of  Figs.  62  and  63  are  enantiomorphous.  Two  forms  are  said  to 
be  complementary  when  their  combination  is  geometrically  indis- 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS  73 

tinguishable  from  another  kind  of  form.  For  example,  the  two 
tetrahedra  of  Fig.  25  are  complementary,  for  geometrically  they 
form  an  octahedron. 

Another  method  of  naming  forms  used  by  some  crystallographers  is  based 
upon  the  position  of  faces  with  respect  to  the  axes  of  reference  (see  below). 

Thus  a  pinacoid  is  denned  as  a  form  that  cuts  one  axis  and  is  parallel 
to  the  other  two.  A  prism  is  defined  as  a  form  that  is  parallel  to  the  vertical 
axis  and  cuts  the  other  two.  A  form  that  is  parallel  to  one  of  the  lateral 
axes  and  cuts  the  other  two  is  a  dome.  A  form  that  cuts  all  three  axes  is 
in  general  called  a  pyramid.  A  pyramid  developed  at  only  one  end  of  the 
verticle  axis  is  known  as  a  hemimorphic  pyramid.  In  the  monoclinic  and 
triclinic  systems  the  names  of  forms  are  based  upon  the  analogy  of  these 
systems  with  that  of  the  orthorhombic.  For  example,  in  the  monoclinic 
system  {hkl}  is  called  a  hemi- pyramid  because  there  are  one-half  as  many 
faces  as  in  the  corresponding  form  of  the  orthorhombic  system;  while  { hkl} 
in  the  triclinic  system  is  a  tetarto-pyramid,  as  there  are  two  faces  instead 
of  eight.  In  the  monoclinic  system  { hOl }  is  a  hemi-dome  because  { hOl  \  in 
the  orthorhombic  system  is  a  dome.  In  the  triclinic  system  JMOJ  is  a 
hemi-prism  consisting  of  two  opposite  parallel  faces  instead  of  the  four 
faces  of  the  prism  {hkO}  of  the  orthorhombic  system.  But,  as  was  said 
before,  the  logical  names  of  forms  are  based  upon  their  symmetry  and 
shape,  and  not  upon  their  position  with  respect  to  the  axes  of  reference. 

6.  THE  NOTATION  OF  CRYSTAL  FACES 

Crystal  measurement  proves  that  exact  mathematical  relations 
exist  between  crystal  faces.  To  make  use  of  this  fact  the  posi- 
tion of  crystal  faces  is  defined  by  the  method  of  analytic  geome- 
try, which  consists  in  referring  them  to  three  (in  one  case,  four) 
suitably  chosen  coordinate  axes  passing  through  the  center  of  the 
crystal.  These  axes  are  sometimes  called  crystallographic  axes, 
but  they  should  be  called  axes  of  reference  to  distinguish  them 
from  axes  of  symmetry.  The  selection  of  these  axes  is  more  or 
less  arbitrary,  but  they  are  chosen  so  as  to  yield  the  simplest 
relations  possible.  Therefore  they  are  usually  either  axes  of 
symmetry,  normals  to  planes  of  symmetry,  or  lines  parallel  to 
prominent  edges. 

Any  face  may  be  defined  by  its  intercepts  on  the  axes  of  refer- 
ence which  in  the  most  general  case  intersect  at  oblique  angles. 
In  Fig.  64  the  axes  are  the  dot-and-dash  lines  OX,  OY,  and  OZ 


74  INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

intersecting  at  the  origin,  0.     The  intercepts  of  the  plane  ABC 

(extension  of  the  face  m)  are  OA,  OB,  and  OC.     The  intercepts 

of  the  plane  HKL  (ex- 
tension of  the  face  n)  are 
OH,  OK,  and  OL.  Now 
it  has  been  found  that 
the  ratios  OA:  OH,  OB: 
OK,  andOC:OLonany 
one  crystal  are  practi- 
cally always  simple 
rational  numbers  such  as 
1:3,  1:2,  1:1,  3:2,2:1, 
B  4:  1,  etc.  This,  the 

«  Y  second  fundamental  law 
of  geometrical  crystallo- 
graphy, is  known  as  the 
law  of  rational  indices 
(Hauy,  1784).  The 
ratios  OA:  OB:  OC  and 

FIG  64  OH:  OK:  OL  are,  on  the 

other  hand,   in  general 

irrational.1     In  Fig.  64  the  ratios  O  A :  OH  =  1:  2,  OB:  OK  =  1:1, 

OC:OL  =  3:2. 

In  the  case  of  the  mineral  barite,  the  relative  intercepts  of  some 

of  the  faces  are  as  follows  (the  letters  refer  to  Fig.  65). 

Face  Relative  intercepts  Weiss  symbols  Miller  symbols 

m  0.815:   1:  oo  d:     b:  oo<}  110 

a  0.815:  00:00  &:*>b:<x>t  100 

u  0.815:  oo  :l. 313  d:  «>b:     6  101 

d  0.815:  oo  :0. 656  d:  oob:^6  102 

I  0.815:  oo  :0. 328  d:  °°b:%6  104 

c  oo     :co:i.313  oo#:  0=6:     6  001 

o  oo     :   1:1.313  ™&\     b:     6  Oil 

y  0.815:0.5:0.656  &:%S:M  122 

z  0.815:     1:1.313  d:     b:     6  111 

'  On  isometric  crystals  even  these  ratios  are  rational  and  on  tetragonal  and  hexagonal 
crystals  two  of  the  three  ratios  are  rational.  This  fact  is  expressed  as  the  law  of  rational 
symmetric  intercepts  (Friedel,  1905). 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS  75 

As  the  expressions  for  the  intercepts  are  cumbersome,  a  very 
simple  method  of  notation  is  suggested  by  the  fact  that  these 
values  for  different  faces  are  in  the  ratio  of  sample  rational 
numbers  (or  infinity) .  We  may  select  the  expression  for  one  of 
these  faces  as  a  standard,  and  represent  the  other  faces  by  the 
numbers  or  infinity.  In  barite  the  face  z  with  the  intercepts 
0.815:  1:1.313  has  been  taken  as  the  unit  face.  This  establishes 
the  axial  ratio  as  d:b:6  =  0.815:  1:1.313. 

The  symbols  for  the  other  faces  may  be  written  as  in  the  third 
column.  These  are  called  Weiss  symbols  from  the  name  of  the 
German  crystallographer  who  proposed  this  method  (1818). 
The  general  expression  for  a  face  in  this  method  of  notation 
is  na:pb:mc,  where  n,  p,  and  ra 
are  simple  numbers  or  fract- 
ions, or  infinity  and  are  called 
coefficients. 

As  the  order  a,  b,  c  is  always 
understood,  these  letters  may  be 
omitted  and  as  infinity  is  incon- 

.          .  ,  ,.      ,        ,      ,  FIG.  65.— Barite  crystal. 

venient  in  mathematical  calcula- 
tions, the  reciprocal  values  of  the  ratios  may  be  used.     We  then 
have  the  symbols  of  the  fourth  column.     If  OA :  OB :  OC  are  the 
intercepts  of  a  unit  face,  the  symbol  of  another  face  with  the  inter- 
cepts OH :  OK :  OL  is  hkl  in  the  expression  OH :  OK :  OL  =  ^  -~ : 

/I         K 

~T  =  h:k'~r  ^e  tnree  smiplest  whole  numbers  h,  k,  I,  that  ex- 
press this  ratio  are  called  the  Miller  indices,  as  Miller,  formerly 
professor  of  mineralogy  at  the  University  of  Cambridge,  was  the 
first  to  make  extensive  use  of  this  method.  The  Miller  symbol 
hkl  is  a  kind  of  algebraic  expression  standing  for  certain  numbers 
and  so  is  called  a  type  symbol.  Besides  a  face  hkl  that  cuts  all 
three  axes  of  reference,  we  have  the  faces  hkO,  hOl,  and  Okl,  each 
of  which  intersects  two  axes  and  is  parallel  to  the  third  and  /iOO 
(100),  OfcO(OlO),  and  002(001)  each  of  which  intersects  one  axis 


76 


INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


and  is  parallel  to  the  other  two.  These  constitute  the  seven  so- 
called  type  symbols.  They  are  represented  in  Fig.  66.  Figure  67 
represents  an  olivine  crystal  with  seven  actual  type  forms  a  { 100} , 
&{010),c{001},w{110},d  {101},fc{021),p{lll}.  As  in  analytic 
geometry,  the  front,  right,  and  top  ends  of  the  axes  are  considered 
as  positive,  while  the  back,  left,  and  bottom  ends  are  considered 
as  negative.  A  negative  index  is  indicated  by  a  line  over  the 
letter.  There  are  eight  planes  which  cut  the  axes  at  the  same 
relative  distances,  but  in  different  octants.  They  are  hkl,  hkl, 
hkl,  hkl,  hkl,  hkl,  hkl,  and  hkl.  These  symbols  as  just  written 


7 


FIG.  66. — The  seven  type  faces. 


FIG.  67. — Olivine  crystal. 


are  face-symbols,  but  the  symbol  of  one  face  hkl  may  be  taken 
to  represent  the  form.  The  form -symbol  is  usually  written 
with  brackets  {hkl}  to  distinguish  it  from  a  face-symbol  hkl  or 
(hkl) .  In  order  to  determine  the  type  symbol  it  is  only  necessary 
to  write  the  indices  h}  k,  I,  in  the  order  of  the  axes  a,  b,  c,  and  to 
substitute  0  if  the  face  is  parallel  to  an  axis.  In  writing  type 
symbols  the  reciprocal  idea  may  be  disregarded  except  for  the 
zero. 

The  determination  of  the  symbol  involves  calculation  by  means 
of   trigonometry,    or    the    corresponding    graphic    solution.     A 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


77 


simple  case  is  illustrated  by  Fig.  68,  which  represents  the  vertical 
prism  zone  of  cerussite.  Here  we  have  a  rectangular  zone  of 
(hkQ)  faces,  a,  m,  r,  b,  where  a  is  (100)  and  b  is  (010),  (the  axes 
of  reference  are  parallel  to  these  two  faces) .  Assuming  m  to  be 
(110),  the  problem  is  to  determine  the  symbol  of  r.  Move  r 
parallel  to  itself  until  r'  and  m'  intersect  the  a-axis  at  a  common 
point.  Then  the  intercept  of  r'  on  the  6-axis,  it  may  be  seen,  is 
one-third  of  that  of  m.  The  intercepts  of  the  r-face  are  la:  lib: 
o°6  or  ^{d:  ^b:  %6.  The  Miller  indices  are  (130)  (read  one, 
three,  zero). 

The  law  of  the  rationality  of  the  indices,  which  has  been  estab- 
lished by  the  measurement 
of  thousands  of  crystals,  is 
the  foundation  of  geome- 
tric a  1  crystallography. 
After  the  axial  ratio  for 
a  given  substance  has 
once  been  established  by 
a  unit  face,  all  the  other 
possible  faces  may  be  pre- 
dicted, for  their  interfacial 
angles  can  be  calculated 
by  the  formulae  of  plane 
or  spherical  trigonometry. 

Why  the  symbol  (111),  in  the  case  of  barite  for  example,  does 
not  represent  a  face  that  cuts  the  three  axes  at  equal  lengths  is 
one  of  the  most  difficult  points  for  the  student  of  crystallography 
to  comprehend.  Several  illustrations  may  clear  up  this  point. 
Imagine  two  cities  laid  out  according  to  different  plans.  In  one, 
the  blocks  are  475  feet  long  and  325  feet  wide,  and  in  the  other 
650  feet  long  and  300  feet  wide.  A  pedestrian  on  inquiring  about 
a  certain  building  in  either  place  might  be  directed  to  go  two 
blocks  north  and  three  blocks  east.  Yet  the  actual  distance  for 
him  to  walk  in  the  two  cities  would  be  different,  for  the  lengths  of 
the  blocks  are  different.  Altho  the  lengths  of  the  blocks  are  on 


c\ 6 £_  - 


FIG.  68. — Graphic  determination  of  indices. 


78          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

record  in  the  city  engineer's  office,  the  pedestrian  is  not  directly 
concerned  with  them,  but  only  with  the  directions  given  him. 
The  axial  ratios  for  crystals  are  established  and  on  record  in 
reference  books,  but  in  the  description  of  the  various  crystal  faces 
and  forms,  use  is  always  made  of  Miller  indices  or  other  symbols 
rather  than  of  the  intercepts  of  the  faces. 

Another  analogous  case  that  will  appeal  to  the  student  of 
chemistry  is  the  law  of  definite  proportions  and  the  law  of  multiple 
proportions.  In  the  chemical  formulae,  CuO  and  Cu2O,  CuO 
means  that  there  are  63.6  parts  (by  weight)  of  copper  and  16 
parts  of  oxygen,  while  Cu20  means  that  there  are  127.2  (2  X  63.6) 
parts  of  copper  and  16  parts  of  oxygen.  The  atomic  weights 
have  been  determined  and  are  given  in  tables,  but  they  are  not 
expressed  in  chemical  formulae. 

In  order  that  the  symbols  may  be  as  simple  as  possible,  it  has 
been  found  convenient  to  have  six  kinds  of  sets  of  axes  of  refer- 
ence; crystals  of  every  known  substance  may  be  referred  to  some 
one  of  these  sets.  The  axes  of  reference  differ  in  their  inclina- 
tions to  each  other  and  the  unit  lengths  on  the  axes  also  differ  in 
their  relative  length.  For  crystals  with  a  single  axis  of  3-fold 
symmetry  or  6-fold  symmetry  it  is  more  convenient  to  use  four 
axes  of  reference  (see  p.  102). 

7.  THE  CLASSIFICATION  OF  CRYSTALS 

The  Crystal  Classes.  The  modern  classification  of  crystals  is 
based  upon  symmetry.  Only  axes  of  2-fold,  3-fold,  4-fold,  and 
6-fold  symmetry  have  ever  been  found  on  crystals.  With  this 
limitation  it  may  be  proved  mathematically  that  only  thirty-one 
combinations  of  symmetry  elements  are  possible.  These  thirty- 
one  divisions  together  with  the  one  division  devoid  of  symmetry 
constitute  the  thirty-two  crystal  classes.  Examples  of  all  of 
these  but  one  (A3.P)  have  been  found  either  among  minerals,  or 
compounds  made  in  the  laboratory.  It  is  interesting  to  note 
that  just  as  Mendele*ef,  the  Russian  chemist,  predicted  the  exist- 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS  79 

ence  and  even  the  properties  of  several  chemical  elements  by 
the  discovery  of  the  periodic  law,  so  Hessel,  a  German  mineralo- 
gist, in  1830  predicted  the  thirty-two  possible  crystal  classes 
when  representatives  of  only  about  half  of  them  were  known. 

The  table  on  page  80  gives  the  name  of  the  class,  the  number 
of  faces  in  the  general  form,'  the  symmetry,  and  a  typical  ex- 
ample. The  name  of  the  form  with  the  symbol  {hkl}  (h-k-h  +  k-l 
in  the  hexagonal  system) ,  or  the  general  form  as  it  is  called,  gives 
the  name  to  the  class.  In  contradistinction,  the  other  forms  are 
called  limit  forms.  This  term  may  be  explained  by  considering 
a  pyramidal  face  (hkl)  in  the  rhombic  bipyramidal  class.  By 
increasing  its  intercept  on  the  vertical  axis  it  becomes  steeper 
and  steeper,  its  limit  in  this  case  being  the  prism  (hkO).  By 
decreasing  its  intercept  on  the  vertical  axis  it  becomes  less  steep, 
its  limit  in  this  case  being  the  pinacoid  (001).  If  its  intercept  on 
the  6-axis  is  increased  it  gradually  passes  into  (hQl),  another 
limit  form,  while  if  its  intercept  on  this  axis  is  decreased  it  becomes 
(010).  Similarly  by  increasing  its  intercept  on  the  a-axis  it 
passes  into  its  limit  (Qkl)  and  then  by  decreasing  its  intercept 
it  becomes  (100). 

The  forms  corresponding  to  the  type  symbols  in  any  class  may  be  found 
from  the  symmetry  by  a  graphical  method.  Indicate  a-  and  &-axes  by  two 
dot-and-dash  lines  at  right  angles  (oblique  angles  in  the  triclinic  system). 
Their  intersection  is  the  projection  of  the  c-axis.  Then  indicate  the  symme- 
try elements  in  their  proper  positions  by  the  following  conventions:  A  full 
line  represents  a  plane  of  symmetry.  A  plane  of  symmetry  parallel  to  the 
plane  of  the  paper  may  be  indicated  by  a  heavy  circle  of  convenient  diameter. 
Denote  axes  of  2-,  3-,  4-,  and  6-  fold  symmetry  by  small  ellipses,  triangles, 
squares,  and  hexagons,  respectively.  As  an  example,  let  it  be  required  to 
find  the  forms  represented  by  the  type  symbols  of  the  rhombic  pyramidal 
class  with  the  symmetry,  A«-2P.  In  Fig.  69  the  two  planes  of  symmetry  are 
represented  by  two  full  lines  which  coincide  with  the  projection  of  the  axes  of 
reference  a  and  b.  Their  intersection  is  the  axis  of  2-fold  symmetry.  Pro- 
jections of  the  faces  in  the  upper  octants  are  small  crosses.  (For  faces  in  the 
lower  octants  circlets  may  be  used.)  Faces  parallel  to  the  vertical  axis  may 
be  indicated  by  arrows.  The  general  form  of  this  class  is  a  rhombic 
pyramid,  for  the  symmetry  requires  (hkl),  (hkl),  and  (hkl)  to  accompany 


80 


INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Table  of  the  Thirty-two  Crystal  Classes 


1 

Faces 

f 

No. 

Name  of  Class 

in 
general 

Symmetry 

Example1 

£ 

form 

V 

gl 

1 
2 

Asymmetric                    1 
Pinacoidal                      2 

No  symmetry. 

c 

(CaS2Q3-6H2O) 
Albite 

jl 

3 
4 

Sphenoidal 
Domatic 

2 
2 

At 
P 

(Sucrose  [sugar]) 
Clinohedrite 

5 

Prismatic 

4 

ArP-C 

Gypsum 

6 

Rhombic   bisphe- 

4 

3AS 

Epsomite 

1    ^ 

noidal 

6)0 

7 

Rhombic  pyram- 

4 

A2-2P 

Calamine 

+3  P 

idal 

O^ 

8 

Rhombic      bipy- 

8 

3A2-3P-C 

Barite 

ramidal 

9 

Tetragonal      bi- 

4 

A2(£>4> 

(Ca2Al2SiO7) 

sphenoidal 

10 

Tetragonal     py- 

4 

A4 

[Ba(SbO)2(C4H4O6)- 

ramidal 

2H2O] 

15 

11 

Tetragonal  scale- 

8 

A2(^P4)-2A2-2P 

Chalcopyrite 

(5 

nohedral 

1 

12 

Tetragonal  trap- 
ezohedral 

8 

A4-4A2 

(NiSO4-6H20) 

•g 

13 

Tetragonal     bi- 

8 

A4-P-C 

Scheelite 

H 

pyramidal 

14 

Ditetragonal  py- 

8 

A4-4P 

(AgF-H2O) 

ramidal 

.     * 

15 

Ditetragonal   bi- 
pyramidal 

16 

A4-4A2-5P-C 

Zircon 

16 

Trigonal  pyram- 

3 

As 

(NaIO4-3H2O) 

idal 

17 

Rhombohedral 

6 

AaC^PsJC 

Phenacite 

/ 

18 

Trigonal  trapezo- 

6 

A3-3A2 

a-Quartz 

hedral 

19 

Ditrigonal     py- 

6 

A3-3P 

Tourmaline 

ramidal 

-20 

Hexagonal  scale- 

12 

A3(JP  6)-3A2-3P-C 

Calcite 

01 

nohedral 

A  *-P 

. 

XI 

22 

Trigonal        bi- 
pyramidal 
Ditrigonal      bi- 

12 

A3  r 

Benitoite 

1 

23 

pyramidal 
Hexagonal      py- 

6 

A8 

Nepheline 

ramidal 

24 

Hexagonal      tra- 

12 

A6-6A2 

/8-Quartz 

25 

pezohedral 
Hexagonal  bipy- 

12 

A«-P-C 

Apatite 

ramidal 

26 

Dihexagonal  py- 

12 

Ae-6P 

lodyrite 

ramidal 

27 

Dihexagonal    bi- 

24 

A6-6A2-7PC1 

Beryl 

pyramidal 

28 

Tetartoidal 

12 

4A3-3A2 

Ullmannite 

•n 

29     Gyroidal 

24                   3A4-4As-6A2 

Sylvite 

"S 

30     Diploidal 

24            4Aj(4iP6)'3A2'3P-C 

Pyrite 

31      Hextetrahedral 

24 

4A  3-3A2(3^P  <)'6P 

Tetrahedrite 

32 

Hexoctahedral 

48 

3A4-4A3(4^>«)-6A2-9P-C 

Galena 

Names  in  parentheses  are  prepared  compounds  of  the  laboratory. 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


81 


\ 


Xhkl 


\hkl 


(hkl).     For   a  face    (Okl),   the  symmetry  requires  (Qkl);  the  form   {0/cZ}, 
then,  is  a  dome.     Similarly  { hOl }  is  a 

dome;  [hkO]  is  a  rhombic  prism;  {lOO}  h^ 

and  JOIO}  are  each  pinacoids,  while 
J001  j  is  a  pedion  consisting  of  a  single 
face.  In  the  lower  half  of  the  crystal 
{hkl}  is  a  rhombic  pyramid;  JO/J), 
and  {hOl},  domes;  while  {OOT}  is  a 
pedion.  The  forms  on  the  lower  half 
of  the  crystal  in  this  case  are  inde- 
pendent of  those  on  the  upper  half. 
Calamine,  an  example  of  a  crystal 
belonging  to  the  rhombic  pyramidal 
class,  is  shown  in  Fig.  70.  Here  the 

forms    are   c{00l),    *{30l[,    i{03lj,  Xhkl        J^nf        Xhkl 

6{010),  rajllO},  and  v{l2l). 


010 


001 


hOl 


Okl 


V 


100 
a 


The     Crystal     Systems,         ^ 
Although  the  thirty-two  classes 
are  fundamental  in  the  classifica-       FIQ-  69.— Graphic  method  of  deter- 

,,  ,    -.      . ,    .  .  mining  the  possible  forms  in  a  crystal 

tion  of  crystals,  it  is  convenient    ciass 

to  assemble  them  in  larger  groups 

called  crystal  systems.  Six  crystal  systems  are  generally  recog- 
nized. It  is  not  always  possible  to  determine  the 
crystal  class  by  inspection,  but  the  crystal  system 
is  usually  apparent  in  well-formed  crystals.  If 
directions  fixed  by  symmetry  (either  axes  of  sym- 
metry or  lines  normal  to  planes  of  symmetry)  are 
chosen  for  axes  of  reference  it  is  found  that  all 
equivalent  faces  are  represented  by  indices  which 
differ  from  each  other  only  in  their  order  of  succes- 
sion and  sign.  If  this  be  done,  one  symbol  (enclosed 
in  brackets)  may  stand  for  all  the  faces  of  a  form. 
The  classification  of  crystals  into  systems  is  largely 
one  of  convenience  to  bring  out  the  relation  of 
crystal  form  to  physical  properties,  but  for  all  prac- 
tical purposes  in  elementary  work  it  may  be  said 

to  rest  upon  the  character  of  the  axes  of  reference  fixed  by  sym- 


Fio.      70 .  - 
Calamine. 


82          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

metry.  Accordingly  the  following  six  systems  are  recognized: 
Isometric,  tetragonal,  hexagonal,  orthorhombic,  monoclinic,  and 
triclinic. 

Crystals  with  three  like  or  interchangeable  directions  of  sym- 
metry at  right  angles  to  each  other  are  referred  to  the  isometric 
system.  These  three  directions,  which  are  either  2-fold  or  4- 
fold  axes  of  symmetry,  constitute  the  axes  of  reference  for  this 
system. 

Crystals  with  a  single  4-fold  axis  (or  a  single  composite  4-fold 
axis,  ^4)  are  referred  to  the  tetragonal  system.  The  other  two 
axes  of  reference,  which  may,  or  may  not,  be  directions  fixed  by 
symmetry,  are  interchangeable  and  are  at  right  angles  to  each 
other  and  also  at  right  angles  to  the  principal  axis. 

Crystals  with  a  single  3-fold  or  6-fold  axis  of  symmetry  (includ- 
ing JP&)  are  referred  to  the  hexagonal  system.  Four  axes  of 
reference  are  usually  employed,  one  (A3  or  Ae)  at  right  angles  to 
three  interchangeable  ones  which  are  in  one  plane  and  intersect 
each  other  at  angles  of  120°.  The  three  lateral  axes  of  reference 
may  or  may  not  be  directions  of  symmetry. 

Crystals  with  three  unlike  or  non-interchangeable  directions 
of  symmetry  at  right  angles  and  no  other  directions  of  symmetry 
are  referred  to  the  orthorhombic  system.  These  three  directions 
are  the  axes  of  reference. 

Crystals  with  a  single  direction  fixed  by  symmetry,  not  pre- 
viously included,  are  referred  to  the  monoclinic  system.  This 
direction  is  an  axis  of  reference;  the  other  two  are  in  a  plane 
normal  to  it  but  are  in  general  oblique  to  each  other. 

Crystals  without  any  directions  fixed  by  symmetry  are  referred 
to  the  triclinic  system.  There  are  three  non-interchangeable 
axes  of  reference,  in  general  at  oblique  angles  to  each  other. 

In  the  case  of  each  of  the  six  systems,  at  least  some  of  the 
directions  fixed  by  symmetry  are  used  for  axes  of  reference. 
If  there  are  not  enough  axes  of  reference,  then  lines  parallel 
to  prominent  edges  or  perpendicular  to  prominent  faces  are 
chosen. 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS  83 

The  forms  of  the  crystal  class  with  the  highest  grade  of  symmetry  in  each 
system  are  sometimes  called  holohedral  or  whole  forms,  while  many  of  the 
forms  of  the  classes  of  lower  grade  of  symmetry  are  called  hemihedral  or 
half  forms,  because  they  have  half  the  number  of  faces  of  the  holohedral 
forms.  There  is  a  geometrical  resemblance  between  these  two  kinds  of 
forms.  A  tetrahedron,  for  example,  is  said  to  be  the  hemihedral  form  of  an 
octahedron,  for  it  may  be  derived  by  extending  alternate  faces  and  suppres- 
sing the  others  as  shown  in  Fig.  71.  A  cube  has  no  hemihedral  form,  or 
rather  the  hemihedral  and  holohedral  cubes  are  geometrically  identical,  for 
the  supression  of  alternate  octants  still  leaves  the  cube.  The  same  is  true 
of  the  rhombic  dodecahedron. 

The  symmetrical  suppression  of  the  faces  of  the  general  forms  of  the 
six  holohedral  classes  gives  rise  to  twenty-six  divisions. 
These,  together  with  the  six  holohedral  divisions,  lead 
to  the  thirty- two  classes  before  mentioned.  The 
general  forms  of  the  five  isometric  classes  may  be 
derived  from  the  hexoctahedron  thus:  The  suppres- 
sion of  faces  of  alternate  octants  gives  the  hextetrahe- 
dron,  the  suppression  of  alternate  faces  gives  the 
gyroid,  the  suppression  of  faces  in  pairs  astride  the 
planes  of  symmetry  gives  the  diploid,  while  the  com- 
bination of  any  two  of  these  methods  gives  a  twelve-  ,  \G '  7.1 '  7  ^  e 
.,,„  ii  1,  derivation  of  the 

sided  figure  called  the  tetartoid.     As  this  form  has     tetrahedron  from  the 
one-fourth  the  number  of  faces  of  the  hexoctahedron,     octahedron, 
it  is  called  a  tetartohedral  or  quarter  form. 

The  idea  of  hemihedrism  implies  that  the  six  crystal  systems  are  funda- 
mental, whereas  we  know  that  the  crystal  classes  are  more  fundamental. 
Hence  the  terms  involving  hemihedrism  are  now  of  historical  interest  only. 
The  class  with  the  highest  symmetry  in  each  system  may  be  called  holo- 
symmetric  instead  of  holohedral. 

8.  CRYSTAL  DRAWING 

Before  the  crystal  systems  and  classes  are  described  in  detail 
the  method  of  drawing  crystals  will  be  explained. 

Crystal  drawings  are  parallel  projections  made  by  drawing 
parallel  lines  from  the  vertices  of  the  crystal  to  the  plane  of 
projection.  If  the  projectors  are  perpendicular  to  the  plane  of 
projection,  we  have  an  orthographic  projection;  if  the  pro- 
jectors are  inclined  to  the  plane  of  projection,  we  have  a  clino- 
graphic  projection. 


84 


INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


The  orthographic  projection  is  especially  useful  in  the  graphic 
determination  of  the  indices  of  crystal  faces  and  axial  elements. 
An  orthographic  projection  is  easily  made  from  the  interfacial 
angles,  without  any  knowledge  of  the  axial  ratios,  simply  by  drop- 
ping perpendiculars  from  the  vertices  of  the  crystal  to  the  plane 
of  projection  which  is  usually  an  actual  or  possible  crystal  face. 

All  faces  normal  to  the  plane  of  the 
drawing  appear  as  lines  inclined  to 
each  other  at  their  true  angles. 
Horizontal  edges  of  the  crystal 
appear  in  their  true  length,  but 
oblique  edges  are  foreshortened. 
Orthographic  projections  lack  the 
appearance  of  solidity  given  by 
clinographic  projections,  but  by 
combining  two  orthographic  projec- 
tions made  on  planes  at  right  angles 
to  each  other,  a  plan  and  elevation 
are  obtained,  which  together  give  a 
good  idea  of  the  crystal  habit.  Fig. 
72  is  a  plan  and  front  elevation  of  a 
topaz  crystal  with  the  forms  c{  001 } , 
2/{041),  ™,{110),  /{120},  w{lll}, 
and  i{  223 } .  The  plan1  (top  figure) 
was  constructed  by  laying  off  the 
interfacial  angles  mm,  ml,  and  II 
and  by  placing  i  and  u  in  the  same 
zone  with  c  and  m.  The  elevation 
(lower  figure)  was  constructed  from  the  interfacial  angle  cy  and 
by  observing  zonal  relations.  The  fact  that  corresponding  points 
in  the  plan  and  elevation  lie  on  the  same  vertical  line  greatly 
facilitates  the  construction.  For  example,  the  directions  of  the 
intersection  edge  ul  in  the  elevation  and  iy  in  the  plan  are  deter- 
mined automatically,  provided  a  supplementary  elevation  is  made. 

1  The  third  angle  projection  is  used. 


FIG.   72. — Plan   and    elevation   of 
topaz  crystal. 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS  85 

From  one  or  two  orthographic  projections  of  a  crystal  it  is 
possible  to  derive  graphically  the  Miller  indices  of  the  faces  and 
the  geometrical  constants  (axial  ratio  and  axial  angles).  Ex- 
amples will  be  shown  under  each  crystal  system.  (See  pages 
97,  101,  111,  115,  120  and  123.)  The  orthographic  projection 
may  be  used  for  graphic  determinations  but  for  general  descrip- 
tive purposes  (text-books  and  articles)  clinographic  projections 
are  preferable,  for  they  give  the  appearance  of  solidity. 

The  clinographic  parallel  projection  or  so-called  parallel  per- 
spective is  used  instead  of  a  true  perspective  because  the  paral- 
lelism of  edges  or  the  occurrence  of  crystal  faces  in  zones  is  one 
of  the  prominent  features  of  crystals  and  should  be  retained  in  the 
drawing.  The  clinographic  projection  is  made  on  a  vertical 
plane  by  inclined  projectors  taken  so  that  one  sees  both  the  top 
and  the  right  side  of  the  crystal. 

The  first  step  in  producing  a  clinographic  projection  of  a  crystal 
is  to  make  an  isometric  axial  cross.  The  method  of  making  an 
isometric  axial  cross  is  shown  in  Fig.  73.  The  upper  right-hand 
part  (a)  of  the  figure  shows  the  rotation  of  the  plan  of  the  axial 
cross  18°  26'  to  the  left.  (This  angle  is  chosen  because  its  tan- 
gent is  J-^).  The  left-hand  portion  of  the  figure  (6)  shows  the 
projection  of  an  elevation  of  the  axial  cross  by  projectors  inclined 
9°  28'  (taken  because  its  tangent  is  J^j)  from  the  horizontal  on  a 
vertical  plane.  The  lower  right-hand  part  of  the  figure  (c)  shows 
the  method  of  obtaining  the  axial  cross  (dot-and-dash  lines). 
The  OC-axis  is  given  in  its  full  length,  but  both  the  OA-axis  and 
the  OB-axis  are  foreshortened. 

The  isometric  axial  cross  is  modified  for  the  other  systems. 
In  the  tetragonal  system  the  unit  length  on  the  vertical  axis  is 
either  greater  or  less  than  that  on  the  lateral  axes.  The  unit 
lengths  of  the  axial  cross  of  a  vesuvianite,  for  example,  are: 
OA:  OB:  0.537  X  OC.  In  the  hexagonal  system  there  are  three 
lateral  axes.  The  method  of  determining  these  lateral  axes 
is  shown  in  Fig.  74.  The  line  OS  is  made  equal  to  1.732  X  OA 
(Fig.  73),  and  S  is  joined  with  B  and  B'.  BS  and  B'S  are  bisected 


86 


INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


at  the  points  Q  and  R.  ROR',  QOQ',  and  BOB'  are  the  unit 
lengths  of  the  three  lateral  axes.  The  unit  length  on  the  vertical 
axis  is  modified  according  to  the  value  on  record.  In  the  ortho- 
rhombic  system  both  the  unit  lengths  on  the  0  A  -axis  and  OC-axis 


FIG.  73. — Construction    of    isometric    axial    cross    in    clinographic    projection 
(modified  from  French). 

are  modified.     For  example,  the  three  values  for  barite  are  0.815 
X  OA  :  OB  :  1.313  X  OC. 

In  the  monoclinic  and  triclinic  systems  the  angles  between  the 
axes  are  also  modified.  For  the  angle  0  between  a  and  c  the 
position  of  the  a-axis  is  changed  as  follows:  On  the  axis  OC  (Fig. 
75)  the  distance  OM  =  cos  0  X  OC  is  laid  off  and  on  the  axis  OA 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


87 


the  distance  ON  =  sin  /?  X  OA  is  laid  off.  Then  the  6-axis  is 
the  line  QOQ' ',  QO  being  the  diagonal  of  a  parallelogram  MQNO. 
The  lengths  of  the  axes  are  modified  as  in  the  other  systems. 


R  \ 


FIG.  74. — Hexagonal  axial  cross.  FIG.  75. — Monoclinic  axial  cross. 


FIG.  76. — Linear  projection  of  a  scapolite  crystal. 

After  the  axes  are  projected  in  their  proper  positions,  crystals 
consisting  of  a  single  form  are  drawn  by  finding  the  intersection 


88 


INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


of  the  faces  on  the  axial  cross  and  connecting  them  with  lines. 
For  crystals  with  two  or  more  forms,  use  is  made  of  the  linear 
projection.  In  the  linear  projection  each  face  is  represented  by  a 
line.  The  lines  are  the  intersections  of  faces,  shifted  parallel  to 
themselves  so  that  they  cut  the  vertical  axis  (c)  at  unity,  with 


FIG.  77. 


no 


100 


110 


010 


FIG.  78.  r*/ 

FIGS.  77-78. — Clinographic  projection  of  a  scapolite  crystal. 

the  plane  of  projection  which  is  a  plane  through  the  center  of  the 
crystal,  perpendicular  to  the  c-axis.  Figure  76  shows  a  linear 
projection  of  the  scapolite  crystal  of  Fig.  78.  It  is  necessary  to 
plot  a  linear  projection  of  the  crystal  on  these  axes  by  taking  the 
reciprocal  of  the  Miller  indices  and  then  making  the  third  term 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS  89 

equal  to  unity.  The  desired  direction  of  the  intersection  edge 
of  the  two  faces  is  a  line  joining  the  intersection  of  the  linear 
projection  of  the  two  faces  with  the  extremity  of  the  vertical 
axis.  Figure  77  shows  the  method  of  construction  of  the  clino- 
graphic  drawing  of  the  scapolite  crystal  of  Fig.  78.  The  dot-and- 
dash  lines  are  the  axes  of  reference;  the  heavy  lines,  the  linear 
projection  constructed  on  the  axial  cross;  while  the  dotted  lines 
are  the  directions  of  the  intersection  edges.  The  direction  inter- 
section of  faces  like  (111)  and  (110),  which  do  not  intersect,  is 
simply  the  direction  of  the  lines. 

9.  ISOMETRIC  SYSTEM 

The  isometric  systems  includes  all  crystals  with  three  inter- 
changeable axes  of  reference  at 
right  angles.  All  crystals  of  this 
system  have  four  axes  of  3-fold 
symmetry;  many  of  them  also 
have  three  axes  of  4-fold  sym- 
metry. 

The  axes  of  reference  are  de- 
signated  as  in  Fig.  79  with  01, 
running  front  and  back,  a2, 
running  right  and  left,  and  a3 
in  a  vertical  position.  As  all 
isometric  crystals  have  identi- 
cal angles  for  corresponding  FlG"  79.-Isometric  axes  of  reference. 

forms,  there  are  no  axial  elements  to  be  determined. 

There  are  five  classes  in  the  isometric  system  (see  p.  80)  but 
of  these  only  the  three  that  are  of  much  practical  importance 
will  be  discussed. 

Isometric  crystals,  unless  much  distorted,  are  of  about  equal 
dimensions  in  all  directions  and  this  fact  aids  in  their  identifica- 
tion. Highly  modified  crystals  may  approach  a  sphere  in 
general  appearance. 


90          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Hexoctahedral  Class.    3A4  4A3(4£>6)  6A2  9P  C 

(Holohedral) 

The  crystals  of  this  class  have  the  maximum  degree  of  symme- 
try possible  in  crystals.  The  4-fold  axes  are  mutually  perpen- 
dicular and  lie  at  the  intersections  of  three  of  the  planes  of 
symmetry  (axial  planes).  The  other  six  planes  of  symmetry 
(diagonal  planes)  intersect  in  the  four  axes  of  3-fold  symmetry. 

The  4-fold  axes  of  symmetry  are  the  axes  of  reference. 

List  of  Forms  in  the  Hexoctahedral  Class 

Cube  6  faces  { 100 } 

Octahedron  8  faces  { 111 

Dodecahedron  12  faces  { 110 

Tetrahexahedron  24  faces  { hkO 

Trisoctahedron  24  faces  { hhl  } 

Trapezohedron  24  faces  { hkk } 

Hexoctahedron  48  faces  { hkl  } 
[In  the  above  symbols  h>k>l] 

Cube  {100}.  The  cube  (or  hexahedron)  is  a  six-faced  form 
with  interfacial  angles  of  90°.  The  ideal  form  is  shown  in  Fig. 
80.  The  cube  is  a  common  form  on  galena,  fluorite,  cuprite, 
and  halite. 

Octahedron  {lllj.  As  its  name  implies,  this  is  an  eight-faced 
form.  Each  face  is  an  equilateral  triangle  in  the  ideal  form. 


FIG.  80.  {100}.  FIG.  81.  {111}.   FIG.  82.  {110}.     FIG.  83.  {hkQ}. 

The  interfacial  angles  are  70°  32'.     (Fig.  81.)     It  is  a  common 
form  on  magnetite,  spinel,  and  diamond. 

Dodecahedron  {110}.  This  form  (Fig.  82)  consists  of  twelve 
faces,  each  rhombic  in  shape.  It  is  often  called  the  rhombic 
dodecahedron  to  distinguish  it  from  the  regular  dodecahedron  of 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS  91 

geometry,  which  is  a  crystallographically  impossible  form.  The 
interfacial  angles  are  60°  and  90°.  It  is  especially  common  on 
garnet. 

Tetrahexahedron  {hkO(.  This  form  is  so  called  because  it 
apparently  consists  of  a  four-faced  pyramid  on  each  cube  face. 
Figure  83  represents  the  form  {210  j .  It  is  occasionally  found  on 
fluorite. 

Trapezohedron  {hkk}.  Each  face  is  a  trapezoid.  This  form 
is  sometimes  called  the  tetragonal  trisoctahedron  to  distinguish 
it  from  the  next  mentioned  form,  the  trigonal  trisoctahedron. 
Figure  84  represents  the  form  {211}  which  is  common  on  garnet, 
leucite,  and  analcite. 

Trisoctahedron  jhhl).  Each  face  is  an  isosceles  triangle. 
With  this  form  the  intercept  on  the  third  axis  is  greater  than  the 


FIG.  84.  \hkk\.       FIG.  85.  [hhl\.        FIG.  86.  [hkl\. 

intercepts  upon  the  other  two,  which  are  equal,  while  with  { hkk } 
the  intercept  on  the  third  axis  is  less  than  the  intercepts  upon  the 
other  two.  Figure  85  represents  the  trisoctahedron  { 22 1  ( ,  a  form 
which  occurs  on  some  crystals  of  galena. 

Hexoctahedron  {hkl}.  The  general  form  of  the  hexoctahedral 
class  consists  of  forty-eight  faces,  the  symbols  of  which  may  be 
derived  from  the  form  symbol  by  taking  six  permutations  of  letters 
and  eight  permutations  of  signs.  Fig.  86  represents  the  hex- 
octahedron  {321(.  It  sometimes  occurs  on  fluorite  crystals  as 
illustrated  in  Fig.  98. 

Combinations.  The  cube,  octahedron,  and  dodecahedron  are 
much  more  common  than  the  other  forms.  They  occur  alone 
and  in  combination  with  each  other.  See  Figs.  423-426,  page  267. 


92 


INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


The  hexoctahedron,  trisoctahedron,  and  tetrahexahedron  usually 
occur  as  small  faces  modifying  simple  forms. 

Galena,  garnet,  fluorite,  and  magnetite  are  given  as  typical 
examples  for  study  and  practice. 


Examples 

Galena.  Usual  forms :  a { 100  j ,  o  { 1 1 1  j .  Interfacial  angles :  aa(100 : 010) 
=  90°0';  oo(lll:  111)  =  70°  32';  oo(100:lll)  =  54°  44'.  Figures  87  to  91 
represent  usual  combinations  varying  from  the  cube  alone  to  the  octahedron 
alone. 


FIG.  87. 


FIG.  88.  FIG.  89.  FIG.  90. 

FIGS.  87-91. — Galena. 


FIG.  91. 


Garnet.  Usual  forms:  d{  110},  n{  211} .  Interfacial  angles:  dd{  110: 101) 
=  60° O';nw(21 1:121)  =  33°33^';/m(211:2ll)  =  48°  ll^';oX110:211)  = 
30°  0'.  Figures  92  to  95  represent  usual  combinations  varying  from  the 
dodecahedron  alone  to  the  trapezohedron  alone. 


FIG.  92. 


FIG.  93.  FIG.  94. 

FIGS.  92-95. — Garnet. 


FIG.  95. 


Fluorite.  Usual  forms :  a  ( 100 ) ,  /  { 3 10 } ,  t  { 42 1 ) .  Cleavage  parallel  to 
{111}.  Interfacial  angles:  oa(100: 010)  =  90°;  a/(100:310)  =  18°  26'; 
<rf(100:421)  =  29°  12'.  Figures  96  to  99  represent  frequent  combinations. 
The  plane  formed  by  the  dotted  lines  in  Fig.  96  represents  octahedral  cleav- 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


93 


age.     Figure  99  represents  a  twin  crystal  in  which. two  cubes  are  twinned 
about  a  cube  diagonal. 


FIG.  99. 


FIG.  96.  FIG.  97.  FIG.  98. 

FIGS.  96-99.  —  Fluorite. 

Magnetite.  Usual  forms:  ojlll},  d(lll),  mJ31l).  Interfacial  angles: 
oo(lll:lll)  =  70°  32';  dd(lW:  101)  =  60°  0';  od  (111:110)  =  35°  16'; 
om(lll:311)  =  29°  30'.  Figures  100  to  102  represent  typical  crystals. 


FIG.  100. 


FIG.  101. 


FIG.  102. 


Hextetrahedral  Class,  4A3-3A2(3-fl>4)-6P 

The  2-fold  axes  are  mutually  perpendicular.     The  planes  of 
symmetry  are  diagonal  to  the  2-fold  axes. 

(Tetrahedral  hemihedral) 

The  axes  of  2-fold  symmetry  (these  are  also  axes  of  composite 
4-fold  symmetry,  ^4)  are  the  axes  of  reference. 


Cube 

Dodecahedron 

Tetrahexahedron 

Tetrahedrons 

Deltohedrons 

Tristetrahedrons 

Hextetrahedrons 


List  of  Forms  in  the  Hextetrahedral  Class 

6  faces  {100J 
12  faces  )  110  } 


24  faces 

hkO 

4  faces 

111 

.  ("I) 

12  faces 
12  faces 

hhl 
hkk 

,  {hhl  J 
},  )hkk| 

[In  the  above  symbols 


24  faces  jhkl  (,  jhkl 


94          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

The  first  four  forms  are  geometrically  different  from  the  cor- 
responding forms  in  the  hexoctahedral  class  and  hence  are 
described  below. 

Tetrahedrons  {111},  {HI}.  This  is  the  regular  tetrahedron 
of  geometry  the  interfacial  angles  being  109°  28'  (Fig.  103). 
The  positive  and  negative  forms  are_  exactly  alike  except  in 
position.  The  two  forms  {111}  and  {111}  in  equal  combination 
form  an  octahedron  geometrically  and  therefore  they  are  said 
to  be  complementary.  The  tetrahedron  occurs  on  tetrahedrite 
and  sphalerite. 

Deltohedrons  jhhl} ,  {hhl} .  These  two  forms  are  also  positive 
and  negative  according  to  the  octant  in  which  they  occur.  Fig. 


FIG.   103.    {111}.      FIG.   104.    {hhl}.        FIG.   105.    \hkk}.      FIG.  106.    {hkl}. 


104  represents  a  positive  form.  The  name  refers  to  the  deltoid 
shape  of  the  faces. 

Tristetrahedrons  {hkk},  {hkk}.  These  forms  resemble  three- 
faced  pyramids  built  upon  each  tetrahedral  face,  hence  the  name, 
tristetrahedron.  The  two  forms,  which  occur  in  alternate 
octants,  are  distinguished  as  positive  and  negative.  Fig.  105 
represents  a  positive  form. 

Hextetrahedrons  {hkl} ,  {hkl} .  The  general  form  is  a  24-faced 
form  called  the  hextetrahedron  as  it  apparently  consists  of  a 
6-faced  pyramid  built  upon  each  face  of  a  tetrahedron.  (Fig. 
106.)  The  two  forms  given  are  distinguished  as  positive  and 
negative.  They  occur  in  alternate  octants. 

Combinations.  Crystals  of  this  class  usually  have  a  tetra- 
hedral aspect.  The  best  example  is  furnished  by  tetrahedrite. 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


95 


Sphalerite  also  belongs  to  this  class,  but  the  crystals  are  usually 
distorted. 

Example 

Tetrahedrite.  Usual^  forms:  o{  111),  o^lTl},  n{ 211),  d{  110} .  Inter- 
facial  angles:  oo(lll:TTl)_=  109°  28';  nn(211:121)  =  33°  33^';  no(211: 
111)  =  19°  28';  do(110:lll)  =  35°.  16'.  Figs.  107  to  110  represent  usual 
types  of  tetrahedrite  crystals. 


FIG.  107. 


FIG.  108.  FIG.  109. 

FIGS.   107-110.— Tetrahedrite. 


FIG.  110. 


Diploidal  Class.    4A3(4£>6)  3A2  3P  C 
(Pentagonal  hemihedral) 

The  planes  of  symmetry  are  mutually  perpendicular.  Their 
intersections  are  the  2-fold  axes.  The  3-fold  axes  are  also 
composite  6-fold  axes. 

The  axes  of  2-fold  symmetry  are  the  axes  of  reference. 

List  of  Forms  in  the  Diploidal  Class 
Cube  6  faces  { 100  [ 

Octahedron  8  faces  {ill} 

Dodecahedron  12  faces  { 110  } 

Pyritohedrons  12  faces  { hkO } ,  { khO } 

Trisoctahedron  24  faces  { hhl 

Trapezohedron  24  faces  { hkk 

Diploids  24  faces  jhkl    ,  {khl} 

[In  above  symbols  h>k>l] 

Of  these  forms,  all  but  the  pyritohedron  and  diploid  are  geo- 
metrically similar  to  those  of  the  hexoctahedral  class. 

Pyritohedrons  {hkO},  jkhO}.  The  pyritohedron  is  so  named 
because  it  is  common  on  the  mineral  pyrite.  The  two  forms 
given  are  arbitrarily  distinguished  as  positive  and  negative.  On 


96 


INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


pyrite  the  most  common  form  is  the  positive  pyritohedron  {210}, 
represented  by  Fig.  111.  The  faces  of  the  pyritohedron  are 
not  regular  pentagons.  A  form  with  twelve  faces  each  a  regular 
pentagon  is  impossible  as  a  crystal  form,  for  it  has  axes  of  5-fold 

symmetry  (see  p.  136). 

Diploids  {hkl},  (khl).  The 
general  form  is  a  24-faced  form, 
the  faces  of  which  lie  in  pairs 
astride  the  planes  of  symmetry, 
hence  the  name,  diploid,  which 
means  double.  The  two  con- 


FIG.  111.    \hkO}.        FIG.  112.    {hkl} 


gruent  forms  {hkl}  and  {khl}  are  distinguished  as  positive  and 
negative.     Figure  112  represents  the  positive  diploid  (321). 
Pyrite  is  the  only  common  example  of  this  class. 

Examples 

Pyrite.  Usual  forms:  a(lOO},  e{210},  o{lll),  s{32l},  n{21l}.  Inter- 
facial  angles:  <ze(100:210)  =  26°  34';  ee(210:210)  =  53°  8';  ee  (210:102)  = 
66°  25';  eo(210:lll)  =39°  14';  oo(100:lll)  =54°  44';  se(321:210)  = 


FIG.  117.  FIG.  118.  FIG.  119. 

FIGS.  113-120.— Pyrite. 


FIG.   120. 


17°  \W\  so(321:100)  =  36°  42';  so(321:lll)  =  22°  12^';  an(100:211)  = 
35°  16';  on(lll:211)  =  19°  28';  Figs.  113  to  120  represent  common  tpyes 
of  pyrite  crystals.  The  cube  faces  are  usually  striated  as  shown  in  Fig.  113. 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


97 


Graphic  Determination  of  Indices  in  the  Isometric  System. 

There  are  no  axial  ratios  to  be  determined  but  simply  the. indices 
of  the  faces.  A  plan  and  elevation  are  made  from  the  inter- 
facial  angles.  Figure  121  represents  pyrite  with  the  faces  a,  e, 
and  o.  The  intercepts  of  the  e-face  in  terms  of  the  unit  are  seen 
to  be  lai  :  2c&2  :  °°a3  which  reduces  to  the  Miller  symbol  (210). 


FIG.   121.— Plan  and  elevation  of  a  pyrite  crystal. 

The  intercepts  of  e\  are:  °°ai  :  J^o2  :  1«3  and  the  indices  (021). 
And  by  means  of  a  side  elevation  the  intercepts  of  the  face  (102) 
could  also  be  determined. 


10.  THE  TETRAGONAL  SYSTEM 

The  tetragonal  system  includes  all  crystals  with  a  single  axis 
of  4-fold  symmetry  (A4)  or  a  composite  4-fold  axis  (JP4)  of 
symmetry  which  is  taken  as  an  axis  of  reference.  The  other  two 
axes  of  reference  are  interchangeable.  The  axes  are  designated 


98          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


GI  :  #2  :  &,  the  unit  lengths  of  ai  and  a2  each  equal  to  unity  and 
the  unit  length  of  6  either  greater 
or  less  than  unity.  The  c-axis 
is  A4  (or  ^4).  Figure  122  repre- 
sents the  axes  for  zircon  and  Fig. 
123,  those  for  apophyllite. 

Of  the  seven  classes  of  the 
tetragonal  system  only  one  is 
treated  here. 


a  2 


FIG.   122. — Tetragonal  axes  of  ref- 
erence for  zircon. 


FIG.  123. — Tetragonal  axes  of  ref- 
erence for  apophyllite. 


Ditetragonal   Bipyramidal   Class.     A4.4A2.5P.C 

(Holohedral) 

The  2-fold  axes  are  normal  to  the  4-fold  axis.  Four  vertical 
planes  of  symmetry  intersect  each  other  at  angles  of  45°,  and  the 
fifth  is  normal  to  these  four. 

The  axis  of  4-fold  symmetry  is  the  c-axis.  As  there  are  four 
axes  of  2-fold  symmetry  at  45°  to  each  other,  either  pair  at  right 
angles  to  each  other  may  be  selected  as  the  lateral  axes. 


List  of  Forms  in 
Pinacoid 
Tetragonal  prism 
Tetragonal  prism 
Ditetragonal  prism 
Tetragonal  bipyramid 
Tetragonal  bipyramid 
Ditetragonal  bipyramid 
[In 


the  Ditetragonal 

2  faces     {  001  } 

4  faces 

100} 

4  faces 

110) 

8  faces 

hkO 

8  faces     jhOl  } 

8  faces 

hhl  } 

16  faces     jhklj 

the  above  symbols 


Bipyramidal  Class 
(Basal  pinacoid) 
(Prism  of  second  order) 
(Prism  of  first  order) 
(Ditetragonal  prism) 
(Pyramid  of  second  order) 
(Pyramid  of  first  order) 
(Ditetragonal  pyramid) 

h>k] 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


99 


Pinacoid  {001}  (Basal  pinacoid).  This  form  consists  of  two 
parallel  faces,  an  upper  one  and  a  lower  one  (Fig.  124). 

Tetragonal  Prism  {100}  (Prism  of  the  second  order).  This  is 
an  open  form  similar  to  {110}  except  in  position  (Fig.  125). 

Tetragonal  Prism  {110}  (Prism  of  the  first  order).  This  is  a-n 
open  form  with  four  faces  euch  parallel  to  the  vertical  axis 
(Fig.  126). 

Ditetragonal  Prism  {hkO[  (Ditetragonal  prism).  An  open 
form  consisting  of  eight  faces,  each  parallel  to  the  vertical  axis 
(Fig.  127) .  The  faces  meet  in  angles  which  are  alternately  equal. 

Tetragonal  Bipyramid  {hOlj  (Pyramid  of  the  second  order). 
A  form  consisting  of  eight  faces  each  parallel  to  one  lateral  axis 
(Fig.  128).  This  form  and  \hhl\  are  identical  except  in  position. 


124(001}.    125(100}.    126(110}.    127{hkO}.    128  {MM}.   129{hhl\.     130{hkl}. 
FIGS.   124-130. — The  seven  type  forms  of  the  ditetragonal  bipyramidal  class. 

Tetragonal  Bipyramid  {hhl}  (Pyramid  of  the  first  order). 
This  form  cuts  the  lateral  axes  at  equal  distances  (Fig.  129).  The 
faces  are  isosceles  triangles  in  the  ideal  form. 

Ditetragonal  Bipyramid  jhkl}  (Ditetragonal  pyramid).  The 
general  form  consists  of  sixteen  faces;  the  faces  in  the  ideal  form 
are  scalene  triangles  (Fig.  130).  The  angles  over  alternate 
polar  edges  are  equal. 

Combinations,  The  bipyramids  are  closed  forms,  but  the 
prisms  and  pinacoids  are  open  forms,  and  hence  must  occur  in 
combination.  In  habit,  tetragonal  crystals  are  usually  prismatic, 
pyramidal,  or  tabular,  but  equidimensional  pseudo-octahedral 
and  pseudo-cubic  crystals  are  not  uncommon. 


100        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Examples 

Zircon,  apophyllite,  and  vesuvianite  are  given  as  typical  examples  for 
study  and  practice. 

Zircon.  6  =  0.640.  Usual  forms:  m{  110},  a{  100),  p{  lll}L  u{33l}, 
x{3ll}.  Interfacial  angles:  mp(110:lll)  =  47°  50';  mm(110:ll0)  =  90° 


m 


m 


FIG.   131. 


FIG.  132.  FIG.  133. 

FIGS.   131-134.— Zircon. 


FIG.   134. 


0';  mo(110:100)  =  45°  0';  a/>(100jlll)  =  61°  40';  ww(110:331)  =  20°  12'; 
xx(3l  1:311)  =  32°  57';  pp(lll:lll)  =  56°  40'.  Figures  131  to  134  rep- 
resent the  usual  combinations  and  habits. 

Apophyllite.     6  =  1.251.     Usual  forms:  a)  100} ,  cjOOl),  p{lll},  j/J310j. 
Cleavage  parallel  to  c{00l).     Interfacial  angles:    co(001:lll)  =  60°  32'; 
ap(100:lll)  =52°  0';   pp(lll:lTl)  =76°  0';   ay(100:310)  =  18°  26'. 
Figures  135  to  138  represent  the  usual  combinations  and  habits. 


<       c 


FIG.  135. 


FIG.  136.  FIG.  137. 

FIGS.  135-138. — Apophyllite. 


FIG.  138. 


Vesuvianite  6  =  0.537.     Usual  forms:  p{  111},  m{  110},  a{  100},  cjOOl}, 
<{33l(,  s{31l}.     Interfacial  angles:  pp(lll:lll)  =  50°39';  cp(001:lll)  = 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


101 


37°  13^';  ap(100:lll)  =  64°  40^';  am(100:110)  =  45°  0>;  m<(110:331) 
23°   41M'j   as(100:311)  =35°   10'.     Figures   139  to  \43  illustr^ ' 
crystals.  '•  *  •• 


m 


FIG.  139. 


FIG.  140. 
FIGS.   139-142.- 


FIG.  141. 
-Vesuvianite. 


FIG.   142. 


Graphic  Determination  of  Indices  and  Axia  Ratio  in  the  Tetra- 
gonal System.  A  plan  and  elevation  of  a  zircon  crystal  are  shown 
in  Fig.  143.  The  unit  bipyramid  { 111 }  is  the  p  face;  the  problem 
is  to  determine  the  symbol  of  u  and  the  axial  ratio  arc.  In  the 
elevation,  lines  parallel  to  the  projections  of  p  and  u  are  drawn 


FIG.   143. — Plan  and  elevation  of  a  zircon  crystal. 

through  the  point  x  to  intersect  the  c-axis.  Then  the  distance 
os  is  equal  to  3  times  the  distance  or.  Therefore  the  symbol  of 
u  is  Io!:la2:3c  or  (331).  The  distance  or  is  equal  to  about  0.64 
of  the  distance  ca\  (in  the  plan) ;  therefore  the  axial  ratio  a  :  c  is 
1  : 0.64.  (It  will  be  noted  that  ox  is  the  foreshortened  ca\.) 


102        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


FIG. 


144. — Hexagonal    axes    of 
reference. 


it.  THE  HEXAGONAL  SYSTEM1 
The  hexagonal  system  includes  all  crystals  with  a  single  axis 

of  3-fold  or  6-fold  symmetry.     (In  two  classes  the  3-fold  axis  is. 

also  a  composite  axis  of  6-fold  symmetry.)  Four  axes  of  refer- 
ence, three  interchangeable  ones  in 
a  plane  at  right  angles  to  the  fourth, 
are  used.  The  positive  ends  of  the 
three  lateral  axes  make  angles  of 
120°  with  each  other,  as  shown  in 

Fig.  144.     The  index  on  the  third 

— • —  axis  is  always  equal  to  the  sum  of 
the  first  two  with  the  sign  changed, 
so  that  the  Miller  symbol  for  the 
general  form  is  {/i-A;-/i+A;-Z},inwhich 
h  is  always  greater  than  k.  The 
axes  may  be  designated  01  :  o2  :  a  a  : 
c,  in  which  the  unit  lengths  on  a\, 
az,  and  a3  are  unity  and  the  unit 

length  on  6  either  greater  or  less  than  unity.     The  axis  of  3-fold 

or  6-fold  symmetry  is  always  taken  as  the  c-axis. 

Dihexagonal  Bipyramidal  Class.     A6-6A2-7P-C 

(Holohedral) 

The  2-fold  axes  are  normal  to  the  6-fold  axes.  There  are  six 
vertical  planes  of  symmetry  at  angles  of  30°  apart.  The  other 
plane  of  symmetry  is  perpendicular  to  these  six. 

List  of  Forms  in  the  Dihexagonal  Bipyramidal  Class 

(Basal  pinacoid) 
(Prism  of  1st  order) 
(Prism  of  2d  order) 
(Dihexagonal  prism) 
(Pyramid  of  1st  order) 
(Pyramid  of  2d  order) 
(Dihexagonal  pyramid) 
[In  the  above  symbols  h>k.] 

1  Classes  16,  17,  18,  19,  and  20  (see  p.  80)  of  the  hexagonal  system  constitute  a  rhombo- 
hedral  subsystem.  They  may  be  referred  either  to  the  four  axes  of  reference  mentioned  or  to 
three  interchangeable  axes  at  equal  oblique  angles  (like  the  legs  of  a  3-legged  stool). 
These  five  classes  are  sometimes  treated  as  a  separate  system,  but  they  are  so  closely 
related  to  the  other  seven  classes  that  they  are  here  retained  in  the  hexagonal  system. 


Pinacoid 
Hexagonal  prism 
Hexagonal  prism 
Dihexagonal  prism 
Hexagonal  bipyramid 
Hexagonal  bipyramid 
Dihexagonal  bipyramid 

2  faces     {0001} 
6  faces     {1010} 
6  faces     J1120} 
12  faces     (h-k-h+k-0[ 
12  faces     jhOhl) 
12  faces     )h-h-2h-l} 
24  faces     {h-k-h+k-l} 

MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


103 


This  form  consists  of  two 
usually    regular  hexagons. 


Pinacoid  {0001J  (Basal  pinacoid). 
opposite  parallel  faces  which  are 
(Fig.  145.) 

Hexagonal   Prism    (1010)    (Prism   of  the  first  order), 
faces  are  in  a  vertical  zone  and  intersect  at  angles  of  60°. 
146.) 


The 

(Fig. 


FIG.  145  {0001}.  FIG.  146  {1010}.   FIG.  147  {1120}.  FIG.  148  [h-k-h+k-Q]. 

Hexagonal  Prism  {1120}  (Prism  of  the  second  order).  This 
form  is  similar  to  {1010}  except  in  position.  (Fig.  147.) 

Dihexagonal  Prism  jh-k-h+k-0}.  All  the  faces  are  in  a 
vertical  zone,  each  being  parallel  to  the  vertical  axis.  Alternate 
angles  are  equal.  (Fig.  148.) 


FIG. 


FIG.  150{h'h-2h'l}. 


FIG.  151{h-k-h+k-l\. 


Hexagonal  Bipyramid  jhOhl)  (Pyramid  of  the  first  order). 
The  faces  cut  two  of  the  lateral  axes,  but  are  parallel  to  the  third. 
(Fig.  149.) 

Hexagonal  Bipyramid  {h  h  2h  1}  (Pyramid  of  the  second  order). 
The  faces  cut  two  of  the  lateral  axes  at  equal  but  greater  dis- 
tances than  the  third  lateral  axis.  (Fig.  150.)  This  form  differs 
from  jhOhl}  only  in  position. 


104        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Dihexagonal  Bipyramid  {h-k-h-fk-1}  (Dihexagonal  pyra- 
mid). This  form  consists  of  24  faces  (scalene  triangles  in  the 
ideal  form),  each  of  which  cuts  the  four  axes  at  unequal  distances. 
The  angles  over  alternate  polar  edges  are  equal.  (Fig.  151.) 

Combinations.  The  habit  is  prismatic,  pyramidal,  or  tabular. 
Simple  combinations  are  the  rule  in  this  class.  As  beryl  is  the 
only  common  mineral  belonging  to  this  class,  it  is  the  only  ex- 
ample given  for  practice. 

Example 

Beryl.  6  =  0.498.  Usual  forms :  c  { OOOlj ,  m  { 10TO } ,  p  { 1011 } ,  s  { 1121 } , 
t>{213l}.  Interfacial  angles:  ?nm(1010:0110)  =  60°  0';  cs(0001:1121)  = 


FIG.  152. 


FIG.  153.  FIG.  154. 

FIGS.  152-155.— Beryl. 


FIG.  155. 


44°  56';  cp(0001:1011)  =  29°  57';  my(1010:2131)  =  37°  49';  ms(1010:1121) 
=  52°  17'.  Figures  152  and  153  are  the  ordinary  combinations.  Figure  154 
has  in  addition  the  general  form  VJ2131).  Figure  155  represents  beryl  of 
tabular  habit,  which  is  rare  as  compared  with  the  prismatic  habit. 

Hexagonal  Scalenohedral  Class.  A8Cfl>6)-3A2-3P-C 

(Rhombohedral  hemihedral) 

The  planes  of  symmetry  intersect  each  other  in  the  3-fold 
axis  and  the  2-fold  axes  are  diagonal  to  the  planes  of  symmetry. 

The  lateral  axes  of  reference  are  the  axes  of  2-fold  symmetry 
and  the  c-axis,  the  axis  of  3-fold  symmetry.  The  3-fold  axis 
is  also  a  composite  6-fold  axis.1 

1  It  is  possible  to  refer  crystals  of  this  class  and  the  next  two  classes  to  three  interchange- 
able axes  of  reference  at  oblique  angles  to  each  other.  In  this  case,  the  Miller  symbol  has 
three  indices  hkl  and  the  axial  element  is  a,  the  oblique  angle  between  the  axes,  which  varies 
for  each  particular  mineral.  (See  footnote  on  p.  102.) 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


105 


List  of  Forms  in  the  Hexagonal  Scalenohedral  Class 


Pinacoid  2  faces 

Hexagonal  prism  6  faces 

Hexagonal  prism  6  faces 

Dihexagonal  prism  12  faces 

Rhombohedrons  6  faces 


{0001} 
{1120} 
{1010} 


{hOhl},  {Ohhl} 


(Basal  pinacoid) 
(Prism  of  2d  order) 
(Prism  of  1st  order) 
(Dihexagonal  prism) 
(Rhombohedrons) 
(Pyramid  of  2d  order) 

(Scalenohedrons) 


Hexagonal  bipyramid!2  faces 
Scalenohedrons          12  faces  jh-k-h+k-l}, 

[In  the  above  symbols  h>k] 

Pinacoid  {0001}  (Basal  pinacoid).  There  are  two  faces  at 
opposite  ends  of  the  vertical  axis.  (Fig.  145.) 

Hexagonal  Prism  {1010}  (Prism  of  the  first  order).     There  are 
six  faces  in  one  zone   meeting  at    angles   of  60°. 
(Fig.  146.) 

Hexagonal  Prism   {1120}    (Prism   of  the  second 
order).     This  form  is    exactly 
like   {1010}  except  in  position. 
(Fig.  147.) 

Dihexagonal  Prism  (h-k-- 
h+kO}.  There  are  twelve 
faces  in  a  vertical  zone.  (Fig. 
148.)  Alternate  angles  are 
equal. 

Rhombohedrons  {hOhl},  {Ohhl}.  A  rhombohedron  consists  of 
six  rhombic  faces,  and  is  like  a  cube  distorted  in  the  direction  of 
one  of  its  diagonals.  A  rhombohedron  is  distinguished  as  acute 
or  obtuse  according  to  whether  the  supplement  angle  over  the 
polar  edges  is  greater  or  less  than  90°.  The  rhombohedron  with 
faces  in  the  middle  front,  right  rear,  and  left  rear  dodecants  is 
called  positive  and  has  the  symbol  {hQhl},  while  the  rhombohe- 
dron with  faces  in  the  right  front,  left  front,  and  middle  rear 
dodecants  is  called  negative  and  has  the  symbol  {Qhhl}.  Figure 
156,  an  obtuse  positive  rhombohedron,  represents  the  cleavage 
rhombohedron  of  calcite. 

Hexagonal  Bipyramid  {h-h-2h-l(  (Pyramid  of  the  second 
order).  This  form  consists  of  twelve  faces,  each  an  isosceles 


FIG.  156   {hOhl}.     FIG.  157  [h-k-h+kt] 


106         INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


triangle.     (Figure   150.)     Hexagonal  bipyramids  are  very  rare 
forms  for  calcite. 


158 


162 


159 


163 


160 


fPS 

m 

P 

m 

164 


167  168 

FIGS.  158-169.— Calcite. 


165 


169 


Scalenohedrons  {h-k-h+k-l(,jk-h-k+h-l}.  The  general  form 
of  this  class  is  a  12-sided  figure,  each  face  of  which  is  a 
scalene  triangle.  There  are  three  kinds  of  edges :  short  polar,  long 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


107 


polar,  and  middle  edges,  each  with  their  characteristic  interfacial 
angles.  The  {h-k-h+k-1}  form  is  called  positive  and  the 
{k-h  k+h-1)  form,  negative.  Figure  157  is  a  positive  scalenohe- 
dron. 

Example 

Calcite.  6  =  0.854._  Usual  forms:'  c{0001},  ra{1010},  a {1120},  e{0ll2}, 
r{10ll},/{0221},  M0332},  M{4041},  VJ2131},  2/13251},  *{2134}L  Cleavage 
parallel  to  r.  Interfacial  angles:  ee(0112: 1012)  =  45°  3';  em(0112:_1010)  = 
63°  45';  rr(10Tl:Il01)  _=  74°  55';  m(1011:10lO)_  =  45°  23^';  #(0221:2021) 
=  101°  9';/w(0221:0110)_=  26° 53'; MM (404 1:4 401)  =  1 14^10' ; Mm (4041: 
lOlO)  =  14°  13';  M(0332:3302)  =  9T  42';  iw(2131:2311)_=  75°  22'; 
tw(2131:3l21)  ^35°  36';  w(2131:1231)  =  47°  1';  ^(3251:3521)  _=  70° 
59';  3^(3251:5231)  =45°  32';  vy (213 1:3251)  ^8°  53';  n>(1011:2131)_  = 
29°  1^';  mv(1010:2131)  =  28°  4';  #(2134:3124)  =20°  36>£';  <e(2134: 
0112)  =  20°  57^'. 

Figures  158  to  169  represent  some  of  the  common  types  of  calcite  crystals. 
The  dotted  lines  in  the  figures  represent  cleavage  planes  which  aid  in  dis- 
tinguishing positive  and  negative  forms. 

Ditrigonal  Pyramidal  Class.  A3-3P 

(Hemimorphic  tetartohedral) 

The  three  planes  of  symmetry  intersect  each  other  in  the  3-fold 
axis  of  symmetry. 

The  lateral  axes  of  reference  are  diagonal  to  the  planes  of 
symmetry. 

List  of  Forms  in  the  Ditrigonal  Pyramidal  Class 

(Basal  planes) 

(Prisms  of  1st  order) 

(Prism  of  2d  order) 

i-o) 

(Ditrigonal  prisms) 

{ hOhl } ,  { hOhl }  (Pyramids  of  1st  order) 

{ Ohhl }  ( Ohhl }  (Pyramids  of  1st  order) 

{h-h-2h-l}       (h-h-2h-l) 

(Hemimorphic  pyramids) 
{h-k-h+k-1}      {h-k-h+k-r-} 

{k-h-k+h-1}     {k-h-k+h-I} 

(Hemimorphic  pyramids) 


Pedions 
Trigonal  prisms 
Hexagonal  prism 
Ditrigonal  prisms 

If  ace       {0001},            {0001} 
3  faces     {1010},           {0110} 
6  faces      {1120} 
6  faces     {h-k-h+k-0}      {k-h-i 

Trigonal  pyramids 
Trigonal  pyramids 
Hexagonal  pyramids 


3  faces 
3  faces 
6  faces 


Ditrigonal  pyramids     6  faces 

Ditrigonal  pyramids    6  faces 
[In  the  above  symbols  h  >k] 


108        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Pedions  {0001},  {0001}.  Each  of  these  forms  consists  of  a 
single  face,  a  positive  pedion  at  the  upper  end  of  the  crystal  and 
a  negative  pedion  at  the  lower  end. 

Trigonal  Prisms  {1010},  {0110}.  These  two  forms  differ  only 
in  position.  Figure  170  shows  {OlTOJ. 


FiG._170 

{0110}. 


FIG.  171 
{h-k-h+k-0}. 


Fio._172 

lohhlj. 


FIG.  173 
{h-k-h+k-1} 


Hexagonal  Prism  {1120},  (Second  order  prism). 

Ditrigonal  Prisms  {h-k-hTk.O},  {k.h.k+h.O}.  The  angles 
over  alternate  angles  are  equal.  Fig.  171. 

Trigonal  Pyramids  {hOEl},  {hOH},  {OhEl},  {Ohhl}  (Hemimor- 
phic  trigonal  pyramids  of  the  first  order).  Each  of  these  forms 
consists  of  three  faces.  They  are  distinguished  as  positive  and 
negative,  and  upper  and  lower.  Figure  172  represents  an  upper 
negative  trigonal  pyramid. 

Hexagonal  Pyramids  {h-h-2h-l},  {h-h-2hl}  (Hemimorphic  hex- 
agonal pyramids).  There  are  six  faces,  each  of  which  cuts  two 
lateral  axes  at  equal  but  greater  distances  than  the  third  lateral 
axis.  

Ditrigonal  Pyramids  {h-k-h+k-lj,  {h-k-h+k-i},  {k-h-k+hl}, 
{k-h-k+h.i}  (Hemimorphic  ditrigonal  pyramids).  The 
general  form  is  a  six-faced  pyramid  with  alternate  angles  equal. 
The  four  forms  indicated  are  the  positive  upper,  positive  lower, 
negative  upper,  and  negative  lower  pyramids.  (Fig.  173.) 

Example 

Tourmaline,  a  complex  boro-silicate,  is  the  best  representative  of  this  class. 

Tourmaline.  jd  =  0.447.  _Usual    forms:    mjlOlpj,    wi{OlTO},   a{ll20J, 

rflOll),   njOlll),    o{022l),    e{0112),    ci{000l},    z{l232).     Interfacial 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


109 


angles:  rr(1011:1101)_  =  46^  52';  rar(1010a011^  =  62°  40';  mo(1010: 
1120)  =  30°  0'  ;  aa(1120  : 1210)  =  60°0'  ;  ee(OlT2  :  T012)  =25°2'  ;  em1(OlT2  : 
OlTO)  :  =75°  30M';  oo(0221  :  2021)  =77°  0'.  (Figs.  174-177.) 


m 


FIG.  174. 


FIG.  175.  FIG.  176. 

FIGS.   174-177. — Tourmaline. 


FIG.  177. 


Trigonal  Trapezohedral  Class.    A3.3A2 

( Trapezohedral  tetartohedral) 

The  2-fold  axes  are  perpendicular  to  the  3-fold  axis.  The 
axes  of  symmetry  are  the  axes  of  reference. 

List  of  Forms  in  the  Trigonal  Trapezohedral  Class 
Pinacoid 
Hexagonal  prism 
Ditrigonal  prisms 
Trigonal  prisms 
Rhomb  oh  edrons 
Trigonal  bipyramids 
Trigonal  trapezohedrons 
Trigonal  trapezohedrons 

[In  the  above  symbols  h>  k.] 

The  two  geometrically  new  forms  for  this  class  are  the  trigonal 
bipyramid  and  trigonal  trapezohedron. 

Trigonal  Bipyramids.  Two  kinds  of  trigonal  bipyramids  are 
possible  for  each  value  of  h.  They  differ  only  in  position.  Fig. 
178  shows  the  form  {h.  h.  2h.l}. 

Trigonal  Trapezohedrons.  The  trigonal  trapezohedron  is  a 
double-ended  6-faced  form  with  the  symmetry  A3.3A2.  Four 


2  faces 
6  faces 
6  faces 
3  faces 
6  faces 
6  faces 
6  faces 
6  faces 

{h-k-h+k-0} 
{1120} 
{hOhl}, 
{h-h-2h-l} 
fh-k-h+k-l} 
{h+k-k-h-l} 

{0001} 
{1010} 
{k-h-k+h-0} 
{1210} 
{Ohhl} 
{2h-h-h-l} 
{k-h-k+h-ll 
{k-h+k-h-l} 

110        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


different  trapezohedrons  are  possible  for  any  given  value  of  h 
and  k.    Figure  179  represents  the  form  {h-k-h+k-l}. 

Example 

Quartz.     6  =  1.099.     Usual  forms:  r{1011},  z{OlTl},  m{10TO},  s{1121|, 

x  {5161} ,  xi  { 6151 } .  Interfacial  angles, 
mm(10TO:OlTO)  =  60°  0'^  wr(10lO: 
1011)  =  38°  13';  mr(0110:1011)  = 
66°  52';  rz(1011:0111)  =  46°  16'; 
rr(10Tl:Tl01)  =  85°  46';_  ms(10lO:- 
1121)  =  37°  58';  _m.T(1010:5161)  = 
12°!';  mxi  (10TO:6l51)  =  12°  1'. 

Figures  180-183  represent  some  of 
the  common  varieties  of  quartz  crystals. 


FIG.   178. 


FIG.  179. 


Graphic  Determination  of  In- 
dices and  Axial  Ratio  in  the 
Hexagonal  System.  Graphic  determinations  in  this  system  are 
illustrated  by  the  plan  and  elevation  of  a  quartz  crystal  shown  in 
Fig.  184.  The  unit  face  r  is  (lOTl),  z  is  (OlTl),  and  m  is  (1010). 
What  is  the  symbol  of  Ml  In  the  side  elevation,  draw  lines 


FIG.  180. 


FIG.  181.  FIG.  182. 

FIGS.   180-183. — Quartz. 


FIG.   183. 


through  n  parallel  to  the  projections  of  the  r  and  M  faces. 
These  intersect  the  vertical  axis  in  the  points  p  and  q.  The 
distance  oq  is  three  times  the  distance  op;  therefore  the  symbol 
of  the  Af-face  is:  \a\ :  o°a2:  ~~ l«s:3c  or  3031. 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


111 


The  axial  ratio  6  (a  =  l)  is  the  distance  op  in  terms  of  oai  (in 
the  plan). 


\ 


FIG.   184. — Plan  and  elevation  of  a  quartz  crystal. 

12.  THE  ORTHORHOMBIC  SYSTEM 

The  orthorhombic  system  includes  all  crystals  with  three  non- 
interchangeable  directions  of  symmetry  at  right  angles  to  each 
other.  The  axial  ratios  are  a:  b:6.  Conventionally  the  unit 
length  of  b  is  unity,  and  the  unit  length  of  a  always  less  than  unity. 


FIG.  185. — Axes  of  ref- 
erence for  topaz. 


FIG.  186.— Axes  of  ref-     FIG.  187.— Axes  of  ref- 
erence for  barite.  erence  for  cerussite. 


These  values  for  the  axial  ratios  differ  for  every  orthorhombic 
substance.  Figures  185,  186,  and  187  represent  the  unit  lengths 
of  the  axes  for  topaz,  barite,  and  cerussite  respectively. 


112        INTRODUCTION  TO  THE"  STUDY  OF  MINERALS 


Of  the  three  classes  of  the  orthorhombic  system,  only  one  is 
discussed  here. 

Rhombic  Bipyramidal  Class.    3A2  3P  C 

(Holohedral) 

The  three  planes  of  symmetry  are  mutually  perpendicular,  and 
their  intersections  are  the  axes  of  2-fold  symmetry.  The  three 
axes  of  2-fold  symmetry  are  the  axes  of  reference.  The  selection 
of  the  c-axis  is  arbitrary,  but  of  the  other  two,  the  unit  on  a  is 
always  shorter  than  the  unit  on  b. 

List  of  Forms  in  the  Rhombic  Bipyramidal  Class 


Pinacoid 
Pinacoid 
Pinacoid 
Rhombic  prism 
Rhombic  prism 
Rhombic  prism 


2  faces  {001} 

2  faces  {010} 

2  faces  {100} 

4  faces  jhkO} 

4  faces  jhOl} 

4  faces  {Okl} 


(Basal  pinacoid) 

(Brachypinacoid) 

(Macropinacoid) 

(Rhombic  prism) 

(Macrodome) 

(Brachydome) 


Rhombic  bipyramid     8  faces     {hkl}      (Rhombic  pyramid)1 

Pinacoid  {001}  (Basal  pinacoid).  This  form  may  be  called 
the  top  pinacoid  (Fig.  188).  The  symbol  is  written  J001J  instead 
of  1 001} ,  for  only  one  form  of  the  kind  is  possible  in  this  class. 

Pinacoid  {010}  (Brachypinacoid).  This  form,  consisting  of 
two  parallel  faces,  one  on  the  right  and  one  on  the  left,  may  be 
called  the  side  pinacoid  (Fig.  189).  The  symbol  is  written  (OlOj 
instead  of  {OfcO}. 

Pinacoid  {100}  (Macropinacoid).  This  form  may  be  called 
the  front  pinacoid  as  it  consists  of  two  opposite  parallel  faces,  one 
in  front  and  one  behind  (Fig.  190).  As  there  is  only  one  pinacoid 
of  this  kind  possible,  the  symbol  {100}  is  used  instead  of  {hOO} . 

Rhombic  Prism  (hkO)  (Rhombic  prism).  An  open  form 
consisting  of  four  vertical  faces  (Fig.  191).  For  each  substance 
crystallizing  in  the  orthorhombic  system  a  whole  series  of  prisms 
is  possible  ranging  from  {010}  to  { 100} .  The  unit  prism  is  { 110} . 

1  These  names  are  used  by  some  authors. 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


113 


Rhombic  Prism  {hOl}  (Macrodome).  A  horizontal  open  form 
composed  of  four  faces  each  parallel  to  the  6-axis  (Fig.  192). 
There  is  also  a  series  varying  from  {001}  to  {010}  for  all  possible 
values  of  h  and  I. 

Rhombic  Prism  {Oklf  (Brachydome).  A  horizontal  open 
form  composed  of  four  faces  each  parallel  to  the  a-axis  (Fig.  193). 
There  is  a  series  of  all  possible  rational  values  of  k  and  I. 


U! 


FlQ.  188.    (001 1          189.    (010 1 


190.    {100}  191.  {hkO}. 

I 


192.{/iOZ{  193.  {Okl}.  194.  {hkl\. 

FIGS.  188-194. — The    seven    type    forms    of    the    rhombic    bipyramidal    class. 

Rhombic  Bipyramid  {hkl}  (Rhombic  pyramid).  The  general 
form  of  this  class  consists  of  eight  faces,  which  in  the  ideal  form 
are  scalene  triangles  (Fig.  194) .  For  any  one  substance  there  is  a 
great  variety  of  forms  possible  depending  upon  various  simple 
rational  values  of  h,  k,  and  I.  If  h  and  k  are  equal  we  have  {hhl} , 
of  which  there  is  a  series  with  varying  values  of  I.  As  these  forms 
are  in  a  vertical  zone  with  the  unit  prism  {110},  they  are  called 
bipyramids  of  the  unit-series.  {111}  is  the  unit  bipyramid. 

Combinations.  Only  the  bipyramids  can  occur  alone.  All 
other  crystals  are  combinations  of  two  or  more  forms.  There 

8 


114        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


are  manifold  combinations  and  consequently  a  great  variety  in  the 
habit.  The  most  common  are  tabular,  prismatic,  and  pyramidal, 
but  some  crystals  cannot  be  placed  under  either  of  these.  Pseudo- 
hexagonal  orthorhombic  crystals  are  common,  but  careful  meas- 
urement distinguishes  them  from  hexagonal  crystals. 

Examples 

Examples  of  orthorhombic  crystals  are  numerous  among  both  minerals 
and  prepared  compounds.  Barite  (BaSOO  and  topaz  (Al2F2SiO4)  are  given 
as  typical  examples  for  study  and  practice  in  working  out  the  forms. 

Barite.  d:b:6  =  0.815:1:1.313.  Usual  forms:  c{001  },  w{110},  &{010}, 
o  {  01  1  }  ,  u  {  101  }  ,  d  {JL02  }  ,  I  {  104  }  .  Cleavage  parallel  to  c  and  m.  Inter!  acial 
angles:  ram(110:lTO)  =78°  22>^';  cw(001:110)  =  90°;  co(001:011)  = 


FIG.  195. 


FIG.  196.  FIG.  197.  FIG.  198. 

FIGS.  195-198.— Barite. 


111 


FIG.  199. 


FIG.  200.  FIG.  201. 

FIGS.  199-202. — Topaz. 


FIG.  202. 


52°  43';  cw(001:101)  =58°  10^';  cd(001:102)  =38°  51';  cZ(001:104)  = 
21°  56'.  Figures  195  to  198  are  usual  combinations.  Figure  65,  page  75,  is 
more  complex  with  a { 100 } ,  z [  111 } ,  and  y  { 122 }  in  addition  to  the  above. 

Topaz.     d:b:6  =  0.528:1:0.477.     Usual  forms:  m{110},  Z{120|,  c{001}, 
/{021 },?/ {041  },u{ 111},  o{221},i {223}.     Cleavage  parallel  to  c.     Interfacial 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


115 


angles;  mmfllO:110}  =55°  43';  K(120:120)  =86°  49';  mZ(110:120)  = 
19°  44';  c/(001:021)  =  43°  39';  cjKOO  1:041)  =  62°  21';  a(001:223)  =  34° 
14';  cw(001:lll)  =  45°  35';  co(001:221)  =  63°  54';  wu(lll:lll)  =  39°  0'; 
oo(221:221)  =  49°  38'.  Figures  199  to  202  represent  usual  types  of  topaz 
crystals.  The  lower  part  of  these  figures  represents  cleavage;  doubly 
terminated  crystals  are  very  rare. 

Graphic  Determination  of  Indices  and  Axial  Ratio  in  the  Ortho- 
rhombic  System.  Figure  203  represents  a  barite  in  plan  and  side 
elevation.  The  unit  faces  are  ra(llO)  and  w(101).  What  are 


FIG.  203. — Plan  and  elevation  of  a  barite  crystal. 

the  axial  ratios  a:b:6t  The  intercept  of  the  m-face  in  the  plan 
gives  us  oa,  which  is  the  unit  length  of  the  a-axis  in  terms  of  ob 
(the  unit  length  on  the  6-axis).  In  the  side  elevation  the  line 
through,  a  parallel  to  the  u  face  determines  the  distance  oc 
which  is  the  unit-length  of  the  c-axis  in  terms  of  ob  of  the  plan. 

7.  THE  MONOCLINIC  SYSTEM 

The  monoclinic  system  includes  all  crystals  in  which  there  is 
a  single  direction  fixed  by  symmetry  not  previously  included  (not 
A3,  A4,  or  Ae).  Three  non-interchangeable  axes  of  reference  are 
used,  one  at  right  angles  to  the  other  two,  which  are  in  general  in- 
clined to  each  other.  The  axial  elements  are  a  :  b  :  6  and  p, 
the  angle  between  the  a-  and  c-axes  (see  Fig.  204).  In  a  few 


116        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

cases  /?  is  equal  to  90°.     The  unit  length  on  the  a-axis  may  be 
either  shorter  or  longer  than  that  on  the  6-axis  which  is  taken  as 

unity.     The  crystal  is  held  so  that 
/  the  a-axis  points  down  and  toward 

j  /  the  observer. 


a/ 


Prismatic  Class.     A2  P-C 
(Holohedral) 

The  axis  of  symmetry  is  normal 
to  the  plane  of  symmetry.  The 
axis  of  2-fold  symmetry  is  the 
6-axis.  The  axes  d  and  6  are  in 

the  plane  of  symmetry,  but  their  position  is  more  or  less  arbitrary. 

They  are  usually  taken  parallel  to  prominent  edges  or  faces. 


FIG.  204. — Monoclinic  axes  of 
reference. 


Pinacoid 
Pinacoid 
Pinacoid 
Pinacoids 
Rhombic  prism 
Rhombic  prism 


2  faces 
2  faces 
2  faces 
2  faces 
4  faces 
4  faces 


{001  } 
{010  } 
{100} 
{hOl  }, 
{hkO} 
{Okl  } 


(Basal  pinacoid) 

(Clinopinacoid) 

(Orthopinacoid) 

(Hemi-orthodomes) 

(Prism) 

(Clinodome) 


Rhombic  prisms     4  faces     {hkl  },   {hkl}      (Hemi-pyramids) 


FIG.  205.       FIG.  206.     FIG.  207.    FIG.  208.  FIG.  209.     FIG.  210.     FIG.  211. 
FIGS.  205-211. — The  seven  type  forms  in  the  prismatic  class. 

Pinacoid  {001)  (Basal  pinacoid).  This  form  is  usually  known 
as  the  basal  pinacoid,  but  its  faces  are  inclined  and  not  perpen- 
dicular to  the  c-axis,  Fig.  205. 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS          117 

Pinacoid  {010}  (Clinopinacoid) .  This  may  be  called  the 
side  pinacoid,  but  it  is  also  known  as  the  clinopinacoid,  Fig.  206. 

Pinacoid  {100}  (Orthopinacoid).  This  form  may  be  called 
the  front  pinacoid,  but  it  is  also  known  as  the  orthopinacoid,  Fig. 
207. 

Rhombic  Prism  {hkO}  (Prism).  An  open  form  consisting  of 
four  faces  each  parallel  to  the  vertical  axis,  Fig.  208. 

Rhombic  Prism  {Okl}  (Clinodome).  An  open  form  consist- 
ing of  four  faces,  each  parallel  to  the  a-axis.  The  a-axis  is  some- 
times called  the  clino-axis  hence  the  name  clino-dome,  Fig.  210. 

Pinacoids  {hOlj,  {hOl}.  (Hemi-orthodomes) .  These  forms, 
each  composed  of  two  opposite  parallel  faces  parallel  to  the  6- 
axis  (often  called  the  ortho-axis),  are  independent  of  each  other. 
Figure  209  represents  IhOl}. 

Rhombic  Prisms  {hkl},  {hkl}  (Hemi-pyramids) .  These  two 
forms  occur  independently,  but  together  they  constitute  a  figure 
that  resembles  a  pyramid;  hence  the  name  hemi-pyramid  is  some- 
times used.  Figure  211  represents  an  {hkl}  form. 

Combinations.  All  monoclinic  crystals  are  necessarily  com- 
binations of  two  or  more  forms,  as  all  the  forms  are  open  ones. 
As  in  the  orthorhombic  system,  the  habits  are  diversified.  If  the 
angle  /?  is  close  to  90°  there  is  often  marked  resemblance  to  ortho- 
rhombic  crystals,  but  this  result  may  also  be  due  to  equal  de- 
velopment of  front  and  back  faces.  Prismatic  crystals  are 
usually  elongated  in  the  direction  of  the  c-axis,  but  occasionally 
in  the  direction  of  the  6-axis,  as  in  the  case  of  epidote,  and  in  the 
direction  of  the  a-axis,  as  in  orthoclase. 

Examples 

Many  minerals  and  also  artificially  prepared  substances  crystallize  in  this 
class.  Orthoclase  (KAlSi3O8),  diopside  (CaMgSi2O6),  augite  (RnSiO3),  and 
gypsum  (CaSO4.2H2O)  are  given  as  good  examples  for  study  and  practice. 
Microcline  is  triclinic,  but  is  so  close  to  the  monoclinic  in  angles  that  it  may 
readily  pass  for  orthoclase. 

Orthoclase.  a:b:t  =  0. 658 :_1: 0.555;  ft  =  63°  57'.  Usual  forms:  c{001}, 
6{010},  m{110|,  z{130},  z{T01},  y{20l],  n{021},  o{Tllj.  Cleavage 


118        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


parallel  to  c  and  b,  also  imperfect  cleavage  parallel  to  m.  Interf  acial  angles  : 
wm(110:ll0)  =61°  13';  &z(01(hl30)^  =_29°  24';  ac(001:I01)  =50°  16'; 
cz/(001  :201)  =  80°  18';  az(edge  HO,  ITO:T01)  =  65°  47'  [a(TOO)  is  a  possible 
face  truncating  the  edge  lTO:TTO];  cn(001:021)  =44°  56';  6o(010:Ill) 
=  63°  8';  6c(010:001)  =  90°  0';  cm(001:110)  =  67°  47'.  Figures  212  to 
215  represent  usual  types  of  crystals. 


FIG.  212. 


FIG.  213.  FIG.  214. 

FIGS.  212-215.— Orthoclase. 


Diopside.  ti:b:6  =  1.092:1:0.589;  0  =  74°  10'.  JJsual  forms:  c{001}, 
6{010},  a{100},  »i{1101,  p{lll},  o{221},  d{101},  A{311|,  a{lll}.  Inter- 
facial  angles:  mm(110:110)  =  92°  50';  o6(10p:010)  =  90°  0';  ac(100:  001) 
=  74°  10^66(010:001)  =  90°  O';_pp(lll:lll)  =  48°  29';_cp(001:lll)  = 
33°  50';  ss(Tll:TTl)  =  59°  11';  oo(221:22D  =  84°  11';  AA(311:311)  =  37° 
50';  cd(001:101)  =  31°  20'.  Figures  216-219  represent  typical  crystals  of 


FIG.  216. 


K 


FIG.  217.  FIG.  218. 

FIGS.  216-219. — Diopside. 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


119 


diopside.     The  striations  on  Fig.  216  are  due  to  polysynthetic  twinning  with 
{001 }  as  twin-plane. 

Augite.  Axial  elements,  usual  forms,  and  interfacial  angles  practically 
the  same  as  for  diopside.  Figures  220  to  223  represent  the  common  types 
of  augite  crystals.  Twins  with  { 100 }  as  twin-plane  are  common  (see  Fig. 
223). 


m 


FIG.  220. 


FIG.  221.  FIG.  222. 

FIGS.  220-223. — Augite. 


FIG.  223. 


Gypsum.     &:b:&  =  0.690:1^0.412;  ft  =  80°  42'.     Usual  forms:  w{110}, 
,    b  {010},    njlll},    e{103}.     Cleavage    parallel    to    6.     Interfacial 
angles:  mm(110:110)  =  68°  30';  6m(010:110)  =  55°  45';  K(lll:lTl)  =  36° 


m 


FIG.  224. 


N/ 

FIG.  225.  FIG.  226. 

FIGS.  224-227. — Gypsum. 


FIG.  227. 


12';  6n(010:ni)_=  69°  20';  W(010:lll)  =  71°  54';  6e(010:103)  =  90°  0'; 
ae(edge  110,110:103)  =  87°  49'.  The  usual  combination  is  bml,  but  with 
varying  habit  as  represented  in  Figs.  224  and  225.  Figure  227  represents  a 
twin  crystal  with  {100}  as  twin-plane. 


120 


INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Graphic  Determination  of  Indices  and  Axial  Elements  in  the 
Monoclinic  System.  An  example  of  graphic  determination  is 
shown  in  Fig.  228,  which  is  a  plan  and  side  elevation  of  an 
orthoclase  (or  microcline)  crystal.  The  unit  faces  are  m(110) 
and  z(101);  the  problem  is  to  determine  the  symbols  of  z  and 
y  and  also  the  axial  elements  a  :  b  :  6  and  /?.  The  a-axis  is  drawn 
parallel  to  c(001)  face  (it  appears  foreshortened  in  the  plan). 
A  line  is  drawn  from  a  in  the  plan  parallel  to  the  z-face.  Its 
intersection  on  the  6-axis  determines  the  distance  os,  which  is 


FIG.  228. — Plan  and  elevation  of  an  orthoclase  (or  microcline)  crystal. 


hence  the  symbol  of  z  is  la  :  >£&  :  « c  which  reduces 
to  (130). 

Similarly  in  the  elevation,  a  line  through  —  a  parallel  to  y 
intersects  the  c-axis  at  the  point,  t.  As  the  distance  ot  =  2  or  (or  is 
the  intercept  of  the  unit  face  x)  the  symbol  of  y  is  —  1  a  :  °°  b  : 
2c  or  (201).  The  symbol  of  o  proves  to  be  111  for  it  is  common 
to  the  two  zones  [010  : 101]  and  [001  :  llO]. 

The  distance  oa  (in  the  elevation)  is  the  unit  length  of  the 
a-axis  in  terms  of  ob  in  the  plan  (the  6-axis),  and  the  distance 
or  in  the  elevation  is  the  unit  length  of  the  c-axis  in  terms  of 
ob  also. 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS          121 


14.  THE  TRICLINIC  SYSTEM 

The  triclinic  system  includes  all  crystals  in  which  there  are  no 
directions  fixed  by  symmetry.     The  axes  of  reference  are  three 
non-interchangeable  axes,  in  general  at  oblique  angles,  which  are 
taken  parallel   to  three  promi- 
nent edges.     The  axial  elements 
are  a  :  b  :  6  (the  unit  on  b  being  6  j 

unity,  and  the  unit  on  a  usually     ^^  \f 

less  than  unity)  and  the  angles  ^~  —  "~^/ 

a,  j8,  and  7  between  the  axes  b  a/>  *^ 

and  c,  a  and  6,  a  and  b  respec-  /  , 

tively.     Figure  229  represents  a 
possible  triclinic  axial  cross. 

The  triclinic  system  includes   _ 

J  FIG.  229.  —  Triclinic  axes  of  reference. 

two  classes,  one  with  a  center  of 

symmetry  and  the  other  without  any  symmetry  whatever. 
As  no  known  mineral  is  devoid  of  symmetry,  only  the  pinacoidal 
class  is  considered  here.  &*ct/±L*,  fa. 


~ 


Pinacoidal  Class.     C 

(Holohedral) 

The  choice  of  axes  is  arbitrary,  but  they  are  usually  taken 
parallel  to  the  intersection  edges  of  the  three  most  prominent 
faces.  In  some  cases,  as  in  the  triclinic  feldspars,  directions 
corresponding  to  those  in  the  monoclinic  feldspar,  orthoclase,  are 
chosen. 

List  of  Forms  in  the  Pinacoidal  Class 

Pinacoid  2  faces  {001}  (Basal  pinacoid) 

Pinacoid  2  faces  {010}  (Brachypinacoid) 

Pinacoid  2  faces  {100}  (Macropinacoid) 

Pinacoids  2  faces  jhkO},  {hkO}  (Hemi-prisms) 

Pinacoids  2  faces  {Okl},  {Okl}  (Hemi-brachydomes) 

Pinacoids  2  faces  {hOlj,  {hOl}  (Hemi-macrodomes) 

Pinacoids  2  faces  {  hkl  }  ,  {  hkl  }  ,  {  hkl  }  ,    {  hkl  }           (Tetarto-pyramids) 


122         INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

All  forms  are  pinacoids  each  of  which  consists  of  two  opposite 
parallel  faces. 

Combinations.  The  appearance  of  triclinic  crystals  depends 
largely  upon  the  obliquity  of  the  axes.  Many  of  them  closely 
approach  monoclinic  crystals  in  angles.  This  is  especially  the 
case  with  the  plagioclase  feldspars. 


£  xamples 

Comparatively  few  minerals  crystallize  in  this  class.  The  only  common 
ones  are  the  plagioclase  feldspars,  rhodonite,  kyanite,  and  microcline. 
Albite  (NaAlSi3O8)  is  selected  as  the  best  mineral 
for  study.  Albite  crystals  are  usually  so  small  that 
measurements  must  be  made  by  the  reflection 
gonometer.  Microcline  is  triclinic  but  is  so  close 
to  orthoclase  in  angles  that  it  may  pass  for  mono- 
clinic.  (The  optical  properties,  especially  the  ob- 
lique extinction  on  the  001  cleavage  face,  prove 
that  it  is  triclinic.) 

Albite.  a:b:6  =  0.633:1:0.556;  a  =93°  58'; 
0  =  63°  39';  7  =  87°  31'.  Usual  forms:  mjllO), 
Mj_110j,  c{001},  6J010},  x[101},  2/{201},  /{130}, 
z{130},  n{021},  p{lll},  ojlll}.  Cleavage  parallel 
FIG.  230.—  Albite.  to  c  and  b.  Interfacial  angles:  mM(110:110)  = 
59°  16^';  m/(110:_130)  =  30°  24',  Mz(110:130)  = 

29°    36';  m6(110:010)  =  60°  58';  cz(001:101)  =  52°  6>^';   c?/(001:201)  = 
81°  53';     Figure  230_represents_an  albite  crystal  with  the  forms:  c{001}, 
and 


Graphic  Determination  of  the  Indices  in  the  Triclinic  System. 
Figure  231  'shows  a  plan  and  side  elevation  of  an  albite  crystal. 
The  unit  faces  are  m(110),  M  (110),  and  #(101)  ;  the  problem  is  to 
determine  the  symbols  of/,  z,  y,  and  p.  (c  =  001;  b  =  010).  The 
determinations  are  made  just  as  they  were  in  the  case  of  Fig.  228; 
the  only  difference  is  that  the  6-axis  is  not  normal  to  the  6(010) 
face.  _The  following  symbols  are  obtained/  =  (130);  z  =  (130); 
2/=  (201);  p  =  (Til). 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS          123 

15.  COMPOSITE  CRYSTALS  AND  CRYSTALLINE  AGGREGATES 

Loose,    isolated   crystals    are   comparatively   rare  in  nature. 
They  usually  occur  in  groups.     The  grouping  may  be  in  parallel 


FIG.  231. — Plan  and  elevation  of  an  albite  crystal. 

position  (see  Fig.  232),  in  the  most  irregular  manner,  or  in  the 
third  condition  of  partial  parallelism. 

Twinning 

The  peculiar  sort  of  grouping  in  partial  paral- 
lelism is  known  as  twinning ;  crystals  so  grouped 
are  called  twin-crystals.  Many  crystals  are 
found  to  be  composed  of  two  parts,  one  half  of 
which  apparently  has  been  revolved  180°  about 
a  line  called  the  twin-axis.  These  may  be 
called  rotation  twins.  Other  crystals  have  two 
portions  symmetrically  placed  with  reference  to 
a  plane  called  the  twin-plane.  These  may  be 
called  reflection  twins.  In  a  third  type  the 
two  individuals  are  symmetrical  to  a  point,  though  neither  of 
the  crystals  has  a  center  of  symmetry.  These  are  called  in- 


FIG.    232— Octa- 
hedra     in     parallel 


124        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


version  twins.  The  face  of  union  of  the  two  individuals  is  called 
the  composition-face.  It  may  or  may  not  be  the  twin-plane. 
The  twin-plane  is  always  a  crystal  face  or  a  possible  crystal 
face,  but  never  a  plane  of  symmetry.  The  twin-axis  is  always 


FIG.  233. 
Contact  twin. 


FIG.  234.  FIG.  235. 

Penetration  twin.       Cyclic  twin. 
FIGS.  233-236. 


FIG.  236. 
Polysynthetic  twin. 


a  possible  crystal  edge  or  a  line  normal  to  a  possible  crystal 
face,  but  it  is  never  a  2-fold,  4-fold,  or  6-fold  axis  of  symmetry. 
Two  general  types  of  twin-crystals  are  distinguished:  (1)  con- 
tact twins  with  a  definite  composition  face  and  (2)  penetration 
twins  with  an  indefinite  or  irregular  com- 
position face.  Figure  233  is  a  diagram- 
matic representation  of  a  contact  twin  and 
Fig.  234,  that  of  a  penetration  twin.  In 
the  case  of  contact  twins  the  twin -law  is 
defined  with  respect  to  a  twin-plane,  while 
in  penetration  twins  it  is  defined  with  re- 
spect to  a  twin-axis. 

In  addition  to  twins  composed  of  two  in- 
dividuals,   there  are   also  multiple   twins 
made  up  of  three  or  more  parts.     If  the 
same  face  serves  as  twin-plane  for  a  series 
of  individuals  we  have  a  polysynthetic  twin  (Fig.  236).     But  if 
different  faces   (of    the   same   form)    are  twin-planes  we  have 
a  cyclic  twin  (Fig.  235). 

A  polysynthetic  twin  may  consist  of  a  large  number  of  in- 
dividuals and  some  of  these  may  be  so  narrow  that  they  appear 


FIG.  237. — Plagioclase. 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


125 


as  striations.  Cleavages  of  calcite  and  of  plagioclase  often  show 
twinning  striations.  In  calcite,  the  rhombohedron  {OlT2}  is 
the  twin-plane,  and  so  on  the  cleavage  face  { 1011}  the  striations 
are  parallel  to  the  long  diagonal  as  represented  in  Fig.  251. 
In  plagioclase  the  twin-plane  is  usually  6{010),  and  so  the  twin 
striations  appear  on  the  c{001f  cleavage  face  as  narrow  bands 
parallel  to  the  (001 :010)  edge  as  shown  in  Fig.  237. 


I       I 


in 


m 


FIG.  238. 
Gypsum. 


FIG.  239. 
Augite. 


m 


\ 


\ 


FIG.  240. 
Hornblende. 


FIG.  241. 
Orthoclase. 


Examples 

Figure  238  represents  a  twin  of  gypsum  with  { 100  {  as  twinning  plane. 
In  Figs.  239  (augite)  and  240  (hornblende),  { 100}  is  also  twin-plane.  Figure 
241  represents  a  Carlsbad  twin  of  orthoclase.  This  is  a  penetration  twin 
with  the  c-axis  as  twin-axis. 

Figu re  242  represents  a  cruciform  penetration  twin  of  staurolite.  In  Fig.  243 
a  contact  twin  of  aragonite  with  ?w{110}  as  twin-plane  is  shown.  A  pene- 
tration trilling  of  cerussite  is  illustrated  by  Fig.  244.  Figure  245,  a  twin  of 
marcasite,  apparently  has  an  axis  of  5-fold  symmetry.  The  angles  in  this 
case  would  be  exactly  72°  (^  of  360°),  but  accurate  measurement  proves 
four  of  them  to  be  74°  55'  instead. 

Figures  246  to  249,  inclusive,  represent  various  kinds  of  rutile  twins,  but  in 
each  case  (101)  is  the  twin-plane.  Figure  246  is  a  simple  contact  twin;  Fig. 
247  shows  twin  striations.  A  single  band  inserted  in  twinning  position 
like  Fig.  248  is  called  a  twin-seam.  Figure  249  is  a  cyclic  twin.  These 
four  figures  are  orthographic  parallel  projections  made  on  the  (100)  plane. 


126        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


The  next  row  of  figures  illustrates  four  of  the  five  twin- 
laws  known  for  calcite.  Figure  250  is  the  scalenohedron  {2131} 
twinned  on  { 0001 } .  Figure  251  represents  a  calcite  cleavage  with 
twin  lamellae  inserted  parallel  to  {0112},  which  is  the  most 


FIG.  242. 
Staurolite. 


FIG.  243. 
Aragonite. 


FIG.  244. 
Cerussite. 


FIG.  245. 
Marcasite. 


common  twin-law  for  calcite.  Figure  252  is  a  calcite  twin  with 
{1011}  as  twin-law,  while  Fig.  253  is  a  scalenohedron  twinned 
on  {0221},  one  of  the  rare  twin-laws  for  calcite. 


m 


FIG.  246. 


FIG.  247.  FIG.  248. 

FIGS.  246-249. — Rutile. 


FIG.  249. 


The  next  four  figures  represent  twins  of  the  isometric  system. 
Figure  254  (with  1 1 1  as  twin-plane)  is  called  the  spinel  twin  be- 
cause it  is  so  common  for  the  mineral  spinel.  Figure  255  repre- 
sents twin  striations  observed  on  cubic  cleavages  of  galena.  Here 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


127 


the  twin-plane  is  the  trisoctahedron  {441}.  A  penetration  twin 
of  fluorite  with  the  cube  diagonal  as  twin-axis  is  represented  in 
Fig.  256,  while  Fig.  257  is  a  twin  of  pyrite  with  the  a-axis  as  twin- 


FIG.  250. 


FIG.  251.  FIG.  252. 

FIGS.  250-253.— Calcite. 


FIG.  253. 


Many  apparently  simple  crystals  are  in  reality  twins.  In 
such  cases  optical  tests  are  usually  necessary  to  reveal  their 
composite  character. 

Twins  are  usually  recognized  by  the  presence  of  reentrant 
angles,  but  there  are  exceptions  to  this  general  rule,  as  for  example, 
hornblende,  Fig.  240,  p.  125. 


FIG.  254. 
Spinel. 


FIG.  255. 
Galena. 


FIG.  256. 
Fluorite. 


FIG.  257. 
Pyrite. 


The  tendency  of  twinning  is  to  raise  the  grade  of  symmetry  apparently. 
This  is  especially  the  case  with  pseudo-hexagonal  orthorhombic  minerals 
such  as  aragonite,  witherite,  and  cerussite.  See  Figs.  472,  473,  474,  and  478. 


128        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Crystalline  Aggregates 

Most  minerals  consist  of  crystal  aggregates  which  do  not 
possess  definite  crystal  faces.  That  such  minerals  are  crystal- 
line, however,  may  be  determined  by  certain  physical  properties, 
particularly  the  optical  properties.  The  kinds  of  crystalline 
aggregates  are  distinguished  by  certain  terms  which  are  con- 
stantly used  in  the  description  of  minerals. 

A  mineral  made  up  of  plates  is  called  lamellar  (example, 
barite).  If  the  layers  are  readily  separated  the  term  micaceous 
is  used  (example,  hematite) .  An  aggregate  of  more  or  less  parallel 
imperfect  crystals  is  called  columnar  (example,  aragonite) ;  and  the 
same  on  a  smaller  scale  is  fibrous  (example,  gypsum).  Fibrous 
radiating  aggregates  of  crystals  are  known  as  spherulites  (ex- 
ample, chalcedony).  A  flat  columnar  aggregate  is  said  to  be 
bladed  (example,  kyanite).  The  term  granular  needs  no  explana- 
tion (example,  magnetite). 

The  forms  assumed  by  many  aggregates  derive  their  names 
from  some  natural  object.  Nodular  is  the  term  used  for  irregular 
rounded  lumps  (example,  pyrite).  Mammillary  refers  to  low 
rounded  prominences  (example,  smithsonite) .  Botryoidal  is 
from  a  Greek  word  meaning  a  bunch  of  grapes  (example,  chal- 
cedony). Reniform  means  kidney-shaped  (example,  hematite). 
The  last  three  terms  are  so  closely  related  that  it  is  often  difficult 
to  decide  which  term  to  use.  The  term  colloform  was  pro- 
posed several  years  ago  by  the  author  for  the  more  or  less  spherical 
forms  assumed  by  amorphous  and  metacolloid  minerals  in  free 
spaces.  Pisolitic  is  the  term  used  for  an  aggregate  of  shot-like 
masses  (example,  cliachite),  while  oolitic  is  similar  except  that  the 
spheres  are  smaller,  about  like  fish-roe  (example,  calcite).  Stalac- 
titic  indicates  that  the  mineral  is  found  in  icicle-like  forms  (ex- 
ample, calcite).  Dendritic  means  branching  like  a  tree  (example, 
copper).  Concretions  are  more  or  less  spherical  masses  formed 
by  the  tendency  of  matter  to  gather  around  a  center  (example, 
siderite).  A  geode  is  a  hollow  concretion  usually  lined  with 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


129 


crystals  (example,  quartz.)     A  vug  is  a  cavity  in  a  rock  or  vein 
lined  with  crystals, 

16.  CLEAVAGE  AND  PARTING 

Many  crystals  have  the  property  of  breaking  with  smooth 
surfaces  in  certain  directions  which  are  parallel  either  to  actual 


FIG.  258. — Cubic  cleavage. 


FIG.  259. — Octahedral  cleavage. 


FIG.  261. — Feldspar  cleavage 


FIG.  262. — Barite  cleavage. 


FIG.  263.— Calcite  cleavage. 


or  possible  crystal  faces.     This  important  property  is  known  as 
cleavage.     (It  really  belongs  among  the  physical  properties  but 


130        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


is  so  intimately  related  to  the  crystal  form  that  it  is  discussed 
here.)  Galena,  which  usually  crystallizes  in  cubes,  has  a  cubic 
cleavage  (Fig.  258),  while  fluorite,  which  also  crystallizes  in  cubes, 
has  an  octahedral  cleavage  (Fig.  259).  (The  octahedron  is  a 
form  sometimes  found  on  fluorite).  Cleavage  is  defined  accord- 
ing to  the  direction  as  cubic,  rhombohedral,  prismatic,  etc.,  and 
according  to  the  character  of  the  surface,  such  terms  as  imperfect, 
good,  perfect,  and  very  perfect  being  used.  Thus  the  micas 
have  a  very  perfect  cleavage  parallel  to  (001)  (Fig.  260),  while  the 
feldspars  have  a  perfect  cleavage  parallel  to 
(001)  and  good  cleavage  parallel  to  (010) 
(Fig.  261). 

In  barite,  an  orthorhombic  mineral,  the 
cleavage  is  perfect  in  one  direction  parallel 
to  (001),  and  a  little  less  perfect  in  two  direc- 
tions parallel  to  (110)  (Fig.  262).  In  gypsum 
there  is  a  very  perfect  cleavage  parallel  to 
(010),  an  imperfect  cleavage  with  conchoidal 
surface  parallel  to  (100),  and  an  imperfect 
cleavage  with  fibrous  surface  parallel  to  (1 1 1) . 
The  relation  of  a  cleavage  fragment  of  gypsum 
to  a  crystal  is  shown  in  Fig.  264.  Here  the 
inner  rhombic  figure  is  the  result  of  cleavage. 
FIG.  264. — Relation  A  line  normal  to  the  paper  is  an  axis  of  two- 
of  cleavage  to  euhedrai  fo^  Symmetry  for  the  cleavage  fragment,  as 

crystal  of  gypsum.  -  J  &         .  & 

well  as  for  the  crystal.  But,  in  general, 
cleavage  shows  a  greater  degree  of  symmetry  than  crystal  form 
for  the  simple  reason  that  the  presence  or  absence  of  a  center  of 
symmetry  cannot  be  established  by  cleavage  alone.  For  example, 
tetrahedral  cleavage  cannot  be  distinguished  from  octahedral 
cleavage.  In  calcite,  whatever  the  shape  of  the  crystal,  the 
cleavage  is  perfect  rhombohedral  in  three  directions  at  angles  of 
74°  55'  to  each  other.  Figure  263  represents  a  cleavage  of  calcite 
with  three  surfaces  and  intersecting  cleavage  traces  on  each. 
Cleavage  is  a  fairly  constant  property  of  minerals,  and  is 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


131 


invaluable  in  the  rapid  recognition  of  minerals.  Such  minerals 
as  calcite,  fluorite,  feldspars,  amphiboles,  and  gypsum  are  dis- 
tinguished principally  by  their  cleavage. 

On  the  other  hand,  such  minerals  as  quartz  and  garnet  possess 
practically  no  cleavage.  They  break  with  an  irregular  fracture. 
In  chalcedony  the  fracture  is  co'nchoidal  (curved  like  the  interior 
of  a  shell).  Other  terms  applied  to  fracture  such  as  splintery, 
hackly,  even,  and  uneven  are  self-explanatory. 

The  term  parting  is  applied  to  a  separation  due  to  some 
molecular  disturbance,  such  as  twinning.  Cleavage  may  be 
obtained  in  any  part  of  a  crystal  in  the  given  direction;  the 
size  and  the  number  of  the  cleavage  particles  are  limited 


FIG.  265. — Halite  parting. 


FIG.  266. — Calcite  parting. 


only  by  the  mechanical  appliance  available.  Parting,  on  the 
other  hand,  takes  place  only  along  certain  definite  planes, 
those  of  the  molecular  disturbance.  In  a  cubic  cleavage  of 
rock-salt,  if  pressure  is  applied  in  a  direction  normal  to  a  vertical 
diagonal  plane,  a  surface  normal  to  the  direction  of  pressure  is 
developed,  (the  shaded  plane  in  Fig.  265) .  In  this  case  we  have 
an  example  of  dodecahedral  parting.  If  pressure  is  applied  by  a 
dull  knife  edge  normal  to  the  obtuse  edge  of  a  cleavage  rhombohe- 
dron  of  calcite,  a  small  portion  of  the  calcite  will  be  reversed  in 
position,  forming  a  twin  with  (0112)  as  twinning  plane.  This 
phenomenon  is  known  as  gliding.  In  this  case  the  small  portion 
of  calcite  in  Fig.  266  may  be  easily  removed;  the  parting  is  par- 


132        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


allel  to  (01 12) ,  a  plane  normal  to  the  direction  of  the  pressure  and  is 
produced  by  secondary  twinning.  In  ice,  gliding  may  take  place 
in  a  direction  normal  to  the  c-axis  (ice  crystallizes  in  the  hexagonal 
system).  This  may  explain  in  part  the  movement  of  glaciers. 

One  case  is  known  in  which  the  direction  of  parting  is  not  a 
possible  crystallographic  plane.  Some  cleavage  pieces  of  plagio- 
clase  show  well-defined  parting  almost,  but  not  quite,  parallel  to 

{001}  See  Fig.  267.  This  parting  is 
due  to  pericline  twinning,  a  method 
of  twinning  in  which  the  6-axis  is  the 
twin-axis.  In  the  triclinic  system, 
if  the  6-axis  is  a  twin-axis,  the  com- 
position face  of  a  twin  is  not  a  possi- 
ble crystal  face. 

Prominent  examples  of  parting  are 
the  following:  basal  parting  (001)  in 
FIG.  267.— Pericline  parting  in   diopside  (Fig.  216,  page  118),  basal 

parting  (001)  in  stibnite,  octahedral 

parting  (111)  in  magnetite,  and  rhombohedral  parting  (0112) 
in  calcite. 

17.  THE  INTERNAL  STRUCTURE  OF  CRYSTALS 

The  law  of  simple  rational  indices  is  the  foundation  stone  of 
geometrical  crystallography.  In  addition  to  the  formulation 
of  the  law  given  on  page  74  it  may  be  stated  in  another  way. 
If  from  a  point,  lines  parallel  to  the  intersection  edges  of  three 
prominent  non-parallel  faces  of  a  crystal  be  drawn,  and  a  plane 
parallel  to  a  fourth  chosen  face  also  be  drawn  there  may  be  con- 
structed from  the  four  points 0,A,B,C  (Fig.  268)  thus  established, 
a  series  of  parallelepipeds  of  indefinite  extent  with  OAGBECDF  as 
a  unit  cell.  Such  a  network  of  points  constitutes  a  space - 
lattice.  The  particular  one  shown  in  Fig.  268  is  triclinic,  the 
distances  OA,  OB,  and  OC  are  unequal,  and  the  angles  between 
them  oblique.  Besides  the  four  planes  mentioned,  many  others 
may  be  drawn  by  connecting  any  three  points  of  the  space-lattice. 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


133 


Now  the  possible  faces  of  a  crystal  are  parallel  to  the  possible 
planes  of  the  space-lattice.  This  is  a  geometrical  expression  of 
the  law  of  rational  indices  without  any  theory  whatever  as  to 
the  internal  structure  of  crystals. 


FIG.  268. — Triclinic  space-lattice. 

A  point  to  be  considered  is  the  limitation  of  the  term  "simple" 
in  the  expression  "law  of  simple  rational  indices. "  All  the  planes 
possible  within  the  limits  of  Fig.  268  are  those  with  simple  indices, 
but  it  is  clear  that  possible  planes  of  the  lattice  may  be  represented 
by  as  large  numbers  as  we  choose  if  the  lattice  is  sufficiently  ex- 
tended. Now  what  are  the  actual  facts?  We  find  that  the 


134        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

crystal  faces  of  common  occurrence  are  those  with  simple  indices : 
0,  1,  2,  3,  4,  5,  and  6,  rarely  above  10.  For  example,  if  we  take 
the  { hhl }  faces  of  all  orthorhombic  minerals  (97  in  number) ,  we  find 
that  those  with  4  as  the  highest  index  occur  on  16  different  min- 
erals, those  with  6  as  the  highest  index  occur  on  5  different 
minerals,  those  with  7  as  the  highest  on  3  minerals,  and  those 
with  9  as  the  highest  on  2  minerals.  Forms  such  as  {9.9.10}, 
{12.12.11},  and  {5.5.19}  occur  on  only  one  mineral.  Geomet- 
rically expressed,  the  faces  of  most  frequent  occurrence  are  those 
of  the  greatest  reticular  density.  This  is  shown  by  Fig.  269  which 


100 


FIG.  269. — Diagram   showing  the  reticular  density  of  {hkO}  faces  in  an  ortho- 
rhombic  crystal. 

may  be  taken  to  represent  some  of  the  possible  hkO  forms  in  the 
orthorhombic  system.  This  fact,  which  is  independent  of  any 
theory,  is  known  as  the  law  of  Bravais. 

Whether  a  crystal  actually  possesses  a  space-lattice  or  not  is 
another  question.  In  1905,  Friedel,  a  French  crystallographer, 
formulated  the  law  of  rationality  of  symmetric  intercepts  (see 
page  74),  which  added  to  the  law  of  rationality  of  indices  practi- 
cally proved  the  existence  of  the  space-lattice.  Direct  proof, 
however,  was  not  furnished  until  the  work  of  Laue  in  1912. 

The  problem  of  crystal  structure  resolves  itself  into  two  more  or 
less  independent  questions:  (1)  the  nature  of  the  constituent 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS  135 

particles  of  the  crystal  and  (2)  their  arrangement  in  space.  The 
second  problem  is  essentially  a  mathematical  one.  It  is  simply 
necessary  to  find  all  the  possible  arrangements  of  points  in  space, 
for  the  constituent  particles,  whatever  their  nature,  may  be 
represented  by  points. 

All  the  possible  space-lattices  -belong  to  one  of  seven  types  of 
symmetry  viz.:  C,  A2  P  C,  3A2  3P  C,  A4-4A2  5P-C,  A3(^>6)-3A2.- 
3PC,  A66A2-7P-C,  and  3A4-4A3(4£>6)-6A2-9P-C.  It  will  be 
noted  that  the  only  symmetry  axes  present  are  those  with  periods 


FIG.  274.  FIG.  275. 

FIGS.  270-275. — Diagrams    showing    possible    axes    of    symmetry    in  a  space- 
lattice. 

of  2,  3,  4,  and  6,  and  the  fact  that  only  these  axes  of  symmetry 
have  been  found  on  crystals  makes  it  probable,  apart  from  any 
other  considerations,  that  crystals  have  a  regular  internal  struc- 
ture. The  diagrams  of  Figs.  270-275  will  make  it  clear  that  only 
these  symmetry  axes  are  possible  in  a  space-lattice.  Let  a\t  0,2, 
03,  etc.  represent  the  points  of  a  space-lattice  and  the  projection 
of  the  lines  of  the  space-lattice.  Let  ai«2  be  the  smallest  possible 
distance  between  them  (the  distances  are  not  infinitesimal). 
Rotation  around  symmetry  axes  of  180°,  120°,  90°,  and  60°  re- 
spectively will  then  give  us  the  points  a3  in  Fig.  270,  as  and  a* 


136    INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

in  Fig.  271,  as,  a*,  and  a5  in  Fig.  272,  and  as,  GU,  as,  a6  and  ai  in 
Fig.  273.  These  figures  show  that  axes  of  2-,  3-,  4-,  and  6-fold 
symmetry  are  possible. 

Next  take  the  case  of  a  5-fold  axis  of  symmetry.  In  Fig.  274  a 
rotation  of  72°  (J£  of  360°)  about  a\  brings  a2  to  as,  and  a  similar 
rotation  about  a2  brings  a i  to  a±.  But  asa4  is  smaller  than  the 
original  distance  aia2,  which  is  contrary  to  hypothesis.  There- 
fore an  axis  of  5-fold  symmetry  is  impossible.  If,  in  a  similar  way, 
we  take  an  angle  of  rotation  less  than  60°  (Fig.  275),  the  new 
points  as  and  a*  result,  but  the  distance  asa4  is  also  smaller  than 
the  original  distance,  a^.  Therefore  axes  of  n-fold  symmetry 
with  n  greater  than  6  are  impossible. 

The  first  substantial  contribution  to  the  subject  of  crystal 
structure  was  made  by  Bravais,  a  French  physicist  in  1850.  He 
found  that  fourteen  space-lattices  are  possible.  They  include, 
in  addition  to  the  seven  primary  lattices  enumerated  below, 

Primary  Space-lattices  Other  Space-lattices 

Cubic  (Fig.  276)  Centered  cube  (Fig.  277) 

Hexagonal  prism  (Fig.  281)  Face-centered  cube  (Fig.  278) 

Rhombohedron  (Fig.  282)  Centered  square  prism  (Fig.  280) 

Square  prism  (Fig.  279)  Centered  rectangular  prism  (Fig.  286) 

Rectangular  prism  (Fig.  285)  Rhombic  prism  (Fig.  283) 

Monoclinic  parallelepiped  (Fig.  288)  Centered  rhombic  prism  (Fig.  284) 

Triclinic  parallelepiped  (Fig.  289)  Clinorhombic  prism  (Fig.  287) 

seven  others  which  may  be  derived  by  combining  two  lattices  in 
a  symmetrical  manner.  For  example,  the  centered  square  prism 
(Fig.  280)  is  made  up  of  two  interpenetrant  square  prism  lattices, 
one  derived  from  the  other  by  shifting  it  in  the  direction  of  its 
diagonal  for  a  distance  equal  to  one-half  of  its  diagonal.  This 
operation  is  known  as  translation.  Now  the  fourteen  space- 
lattices  of  Bravais  are  the  only  possible  ones  that  may  be  ob- 
tained by  adding  translations  to  the  symmetry-operations, 
provided  no  translations  less  than  one-half  the  distance  between 
the  points  of  a  primary  lattice  are  used.  Every  Bravais  space- 
lattice  necessarily  has  a  center  of  symmetry. 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS  137 


FIG.  276. 


Fro.  277. 


FIG.  278. 


FIG.  279. 


FIG.  280. 


FIG.  281.  FIG.  282. 


FIG.  283. 


FIG.  284. 


FIG.  285. 


FIG.  286. 


FIG.  287.  FIG.  288.  FIG.  289. 

FIGS.  276-289.— The  fourteen  space-lattices  of  Bravais. 


138        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Sohncke  in  1879  discovered  a  new  kind  of  symmetry  opera- 
tion applicable  to  points  in  space,  viz.,  a  screw -axis  of  symmetry, 
which  is  an  axis  around  which  a  spiral  movement  takes  place.  It 
may  be  produced  by  combining  an  ordinary  axis  of  symmetry 
with  a  translation  along  the  direction  of  the  axis.  An  example  of 
a  4-fold  screw-axis  of  symmetry  is  shown  in  Fig.  290.  The 
point  b  may  be  derived  from  a,  c  from  b,  d  from  c,  a^  from  d,  and 
so  on,  by  a  clockwise  rotation  of  90°  combined  with  a  translation 
t,  equal  to  J^  aai.  A  screw-axis  may  be  either  right-handed  or 


FIG.  290. — Screw-axis  of  symmetry.       FIG.  291. — Glide-plane  of  symmetry. 

left-handed;  it  is  thus  possible  to  account  for  exactly  similar,  but 
enantimorphous,  crystals  such  as  those  of  quartz  (see  Figs.  420-21, 
p.  260).  By  adding  screw-axes  to  the  14  Bravais  space-lattices, 
Sohncke  proved  that  57  kinds  of  arrangements  of  points  are 
possible.  These  are  called  point-systems. 

It  was  soon  realized  that  Sohncke's  work  was  incomplete, 
and  the  Russian  crystallographer  Fedorov  a  little  later  found 
that  a  new  type  of  symmetry  was  necessary,  viz.  a  glide-plane 
of  symmetry,  which  is  the  result  of  combining  reflection  in  a 
plane  of  symmetry  with  a  translation  parallel  to  the  plane.  An 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS  139 

example  is  shown  in  Fig.  291.  The  dotted  lines  are  the  traces  of 
a  glide-plane  of  symmetry,  for  with  a  translation  equal  to  t,  this 
plane  becomes  a  plane  of  symmetry.  (A  tessellated  pavement 
furnishes  a  good  example  in  two-dimensional  space) .  The  result 
of  combining  translations,  screw-axes,  and  glide-planes  with  the 
ordinary  symmetry-operations  of  finite  figures  furnished  Fedorov 

(1890)  with  230  possible  kinds  of  space-groups.     Each  of  these 
space-groups  belongs  to  one  of  the  32  point-groups.     Schoenflies 

(1891)  and  Barlow  (1894)   arrived  at  the  same  conclusion  in- 
dependently.    The  mathematical  side  of  the  problem  of  crystal 
structure  was  thus  firmly  established,  and  Friedel's  recognition  of 
the  law  of  the  rationality  of  symmetric  intercepts  in  1905  practi- 
cally proved  that  a  space-lattice  exists  in  crystals. 

Visible  proof  of  the  space-lattice,  however,  was  not  forthcoming 
until  1912.  The  results  obtained  in  the  next  few  years  have 
opened  up  one  of  the  most  interesting  fields  in  the  whole  realm 
of  science. 

The  first  experimental  work  on  crystal  structure  originated  in 
an  attempt  to  determine  the  nature  of  .X-rays.  It  was  doubtful 
whether  X-rays  consisted  of  a  wave-motion  or  were  material  in 
nature.  Laue,  a  Swiss  physicist,  conceived  the  idea  that  a 
crystal  might  act  as  a  3-dimensional  diffraction  grating  for 
X-rays  if  the  latter  consisted  of  a  wave-motion.  In  order  to 
test  this,  a  beam  of  X-rays  was  directed  upon  a  crystal  plate  of 
sphalerite  (isometric  ZnS),  and  a  photograph  was  obtained  which 
showed  a  central  circular  spot  surrounded  by  elliptical  spots  of 
varying  intensity  arranged  in  a  symmetrical  manner.  Two 
radiograms  (X-ray  photographs)  of  sphalerite  are  shown  in  Figs. 
292a  and  2926.  Fig.  292a  was  taken  from  a  crystal  plate  cut 
parallel  to  a  cube  face  and  Fig.  2926  from  a  plate  cut  parallel 
to  a  tetrahedral  face. 

The  experiment  had  succeeded  and  proof  was  furnished  that 
the  mysterious  X-rays  discovered  by  Roentgen  are  the  result  of 
a  wave-motion  similar  to,  but  with  much  shorter  wave-length 
than,  that  of  light.  The  central  spot  of  the  photograph  is  pro- 


140        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


FIG.  292a-6. — Radiograms  of  sphalerite  (after  Frederick  and  Knipping), 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


141 


duced  by  the  direct  beam  of  X-rays  and  the  other  spots  are  due 
to  secondary  beams  produced  by  reflections  from  internal  planes 
of  particles.  (The  actual  particles  are  of  course  too  small  to 
show  separately,  for  millions  of  them  are  present  even  in  a 
minute  crystal.)  The  fact  that  the  intensity  of  the  spots  is  pro- 
portional to  the  reticular  density  of  the  plane  just  as  the  relative 
frequency  of  occurrence  of  crystal  forms  makes  it  possible  to 
determine  the  actual  position  of  the  constituent  particles. 

This  work  has  been  undertaken  by  a  number  of  investi- 
gators, notably  the  English  physicist,  W.  H,  Bragg,  and  his 
son,  W.  L.  Bragg.  The  Braggs  introduced  a  method  different 
from  that  of  Laue.  They  mounted  the  crystal  upon  an  X-ray 


FIG.  293. — Explanation  of  X-ray  diffraction  effects  (after  the  Braggs). 

spectrometer,  an  instrument  in  which  the  telescope  of  the  ordi- 
nary spectrometer  is  replaced  by  an  ionisation  chamber,  and  used 
a  beam  of  X-rays  of  given  wave-length  analogous  to  a  beam  of 
monochromatic  light.  (In  the  Laue  experiments  the  general 
radiation  corresponding  to  white  light  was  used.)  As  the 
mounted  crystal  is  revolved  about  the  axis  of  the  spectrometer, 
effects  are  obtained  over  a  considerable  range  of  angles.  At 
certain  angles  the  leaf  of  the  electroscope  attachment  moves  and 
the  intensity  of  the  movement  may  be  read  off  on  a  suitable  scale. 
The  reason  that  the  leaf  of  the  electroscope  moves  only  at  certain 
angles  may  be  shown  by  a  consideration  of  Fig.  293.  The 
horizontal  lines  pp,  etc.,  represent  traces  of  internal  planes  in  the 
crystal  with  the  spacing  d.  A,  A\,  Az,  A*.  .  .  .are  a  train  of 


142        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


100 


X-ray  waves  of  wave  length  X.  (It  is  to  be  noted  that  d  and 
X  are  of  the  same  order  of  magnitude  which  makes  it  possible  to 
use  a  crystal  as  an  X-ray  diffraction  grating.)  The  reflected 
wave  BC  is  the  result  of  reflections  from  successive  planes  B, 
B',  B" ',  B'",  etc.,  provided  the  distance  ND  is  equal  to  X  or  a 
multiple  of  X,  for  then  all  the  trains  of  reflected  waves  are  in  the 
same  phase  and  the  intensity  is  equal  to  the  sum  of  their  ampli- 
tudes. If  the  distance  ND  is  not  equal  to  X  or  n\  there  is  no  re- 
flection. It  is  clear  then  that  reflection  takes  place  only  at  certain 
angles  X  =  2d  sin  0i;  2X  =  2d  sin  02;  3X  =  2d  sin  63;  etc.  (ND  = 
2d  sin  B) .  For  a  different  crystal  face  the  value  d  will  be  differ- 
ent and  consequently  the  angle  6.  The  readings  obtained  can 

then  be  used  to  interpret  the 
crystal  structure.  The  re- 
sults obtained  by  the  Braggs 
for  sphalerite  are  shown  in 
Fig.  294  for  the  three  im- 
portant faces  of  the  isometric 
system  viz.,  the  cube  {100}, 
**  »* ^  M* 6b°  the  dodecahedron  { 1 10} ,  and 

FIG.     294.-X.ray    spectrometer    readings   the   octahedron   {ill}.      For 
for  sphalerite  (after  the  Braggs). 

the  octahedral  or  tetrahedral 

plane  there  are  four  readings,  corresponding  to  spectra  of  the 
first,  second,  third,  and  fourth  order  of  the  diffraction  grating. 
Now  there  are  three  different  kinds  of  space-lattice  possible  in 
the  isometric  system:  (1)  the  cube  (Fig.  276),  the  centered 
cube  (Fig.  277),  and  the  face-centered  cube  (Fig.  278).  The 
distance  of  adjacent  reticular  planes  in  the  three  cases  for  the 
prominent  faces  are  as  follows: 

1          1          1 


no 


in 


Cube  lattice 


Centered  cube  lattice 


:  V3 


1 


Face-centered  cube  lattice 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS 


142 


The  ratios  of  the  sines  of  the  angles  for  sphalerite  given  in  Fig. 
294  are  practically  as  l:\/2:\/f>  which  proves  that  the  con- 
stituent particles  of  a  sphalerite  crystal  lie  at  the  points  of  a 
face-centered  cubic  lattice.  The  Braggs  conclude  that  equally 
spaced  planes  parallel  to  the  cube  contain  Zn  and  S  atoms  alter- 
nately (Fig.  295),  that  equally  spaced  planes  parallel  to  the 
dodecahedron  contain  both  Zn  and  S  atoms  (Fig.  296),  and  that 
unequally  spaced  planes  parallel  to  the  octahedron  (or  tetra- 
hedron) contain  Zn  and  S  atoms  alternately,  also  that  the  Zn-Zn 
distance  is  four  times  the  Zn-S  distance  (Fig.  297).  The 
internal  structure  of  sphalerite  must  then  be  that  shown  in  Fig. 
298.  (The  lines  of  the  figure  are  of  course  imaginary.  They  are 


i 
1 
I 
1 
1 
i 

1 
1 
1 
1 

n      S      2 

n     S     Z 

n           2 

n          Z 
3           S 

n           2 

> 

rn          Z 

5           < 

n        Z 

iS          Z 

nS        Zn    i 

F 

to.  295. 

FIG.  2< 

)6. 

FIG 

.  297. 

FIGS.  295-297. — Spacing  of  planes  of  atoms  in  sphalerite  (after  the  Braggs). 

drawn  simply  to  show  the  relation  of  the  atoms  to  each  other) . 
The  zinc  atoms  lie  on  a  face-centered  cubic  lattice  and  the  sulfur 
atoms  on  another  face-centered  cubic  lattice  distant  one-fourth 
of  the  diagonal  of  the  cube  apart.  The  symmetry  elements  of  the 
structure  are  as  follows:  There  are  dodecahedral  planes  of 
symmetry  through  the  Zn  and  S  atoms  at  intervals  of  J^A/2a, 
where  a  is  the  dimension  of  the  edge  of  the  unit  cube,  and  halfway 
between  these,  glide-planes  of  symmetry  with  a  translation  of 
HVf  a  in  the  direction  between  the  central  Zn  and  the  lower 
front  right  Zn.  Lines  normal  to  the  cube  planes  at  the  projection 
of  all  the  zinc  and  sulfur  atoms  are  composite  planes  and  four- 
fold axes  of  symmetry  with  the  planes  at  intervals  of  Ha.  There 
are  also  two-fold  screw-axes  of  symmetry  with  translation  of  %a 


144        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Pio.  298. — The  internal  structure  of  sphalerite. 


MORPHOLOGICAL  PROPERTIES  OF  MINERALS  145 

along  vertical  and  horizontal  lines  half-way  between  vertical  and 
horizontal  lines  of  Zn-atoms.  No  centers  of  symmetry  are  pres- 
ent for  the  three-fold  axes  of  symmetry  lying  along  -the  cube 
diagonals  are  polar.  The  space-lattice  shown  in  Fig.  298  thus 
accounts  for  the  symmetry  of  sphalerite:  4A3-3A2(3^P4)-6P. 

The  Laue  radiogram  of  Fig.  -292a  apparently  shows  four  planes 
of  symmetry  (axial  as  well  as  diagonal)  intersecting  in  a  central 
axis  of  4-fold  symmetry  but  Friedel  has  proved  that  a  radiogram 
does  not  tell  us  whether  a  center  of  symmetry  is  present  or  not. 

The  unit  cube  of  the  sphalerite  structure  contains  4  sulfur 
atoms  and  4  zinc  atoms,  for  the  zinc  atoms  at  the  8  corners  each 
belong  equally  well  to  the  seven  adjacent  cubes  and  each  of  those 
at  the  centers  of  the  cube  faces  belong  equally  well  to  an  adjacent 
cube.  [(8  X  M)  +  (6  X  }4)  =  4].  This  does  not  mean  that  the 
molecular  formula  of  sphalerite  is  4  ZnS  for  it  is  probable  that  the 
molecule  does  not  exist  in  crystalline  solids,  but  only  in  gases, 
liquids;  and  amorphous  solids. 

The  structure  of  diamond  is  similar  to  that  of  sphalerite  except 
that  all  the  points  of  Fig.  298  are  carbon  atoms.  The  dodeca- 
hedral  planes  are  planes  of  symmetry  with  intervals  of  J4\/2a 
and  the  cube  planes  at  intervals  of  Y±a  are  glide-planes  of  sym- 
metry with  translation  of  J^a  along  a  line  normal  to  a  cube  edge. 
Composite  four-fold  axes  of  symmetry  are  at  the  projections  of  all 
the  carbon  atoms  and  corresponding  to  the  two-fold  screw-axes  of 
Fig.  298  we  have  instead,  four-fold  screw-axes  with  translations 
of  y±a  in  the  direction  of  the  cube  edges.  The  three-fold  axes  of 
symmetry  are  no  longer  polar  as  in  the  case  of  sphalerite.  The 
work  of  the  Braggs  proves  that  diamond  belongs  to  the  hexocta- 
hedral  or  holosymmetric  class  of  the  isometric  system. 

In  practically  all  of  the  discussions  of  crystal  structure  the 
idea  of  a  unit  is  prominent.  The  term  " particle"  often  is  used 
instead  of  molecule  because  of  the  possibility  that  the  unit  of 
structure  is  a  collection  of  molecules  instead  of  a  single  molecule. 
Some  interpret  the  ^T-ray  work  of  the  Braggs  to  mean  that 

molecules  as  such  do  not  exist  in  crystals,  but  in  the  opinion  of 
10 


146        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

others  this  conclusion  is  premature.  The  molecule,  if  such  exists, 
may  escape  detection  by  X-ray  methods.  In  any  event  the 
X-ray  experiments  prove  that  not  only  the  units  of  structure 
but  also  the  individual  atoms  are  arranged  in  space-lattices  which 
was  predicted  by  Groth  in  1905.  The  sphalerite  crystal  of  Fig. 
298,  for  example,  is  built  up  of  two  interpenetrant  space-lattices, 
one  of  Zn  atoms  and  the  other  of  S  atoms.  The  nature  of  the 
unit  of  crystal  structure  still  remains  in  doubt. 

The  X-ray  analysis  of  crystals  combined  with  a  mathematical 
study  of  the  possible  arrangements  of  points  in  space  has  fur- 
nished us  with  a  means  of  determining  the  stereochemistry  of  the 
solid  or  crystalline  state  and  has  thus  thrown  new  light  on  the 
structure  of  matter.  This  work  also  promises  to  be  of  value 
in  settling  many  doubtful  questions  concerning  crystals.  It 
has  truly  opened  up  one  of  the  most  interesting  fields  in  the 
whole  realm  of  science. 


THE  PHYSICAL  PROPERTIES  OF  MINERALS 

Two  classes  of  physical  properties  are  recognized.  Physical 
properties  such  as  specific  gravity  are  independent  of  the  direction 
and  are  called  scalar  properties,  while  others,  such  as  cohesion 
and  the  effect  of  light,  heat,  and  electricity,  can  be  represented  by  a 
line  of  given  length  and  direction,  hence  the  term  vectorial 
properties. 

Vectorial  properties  may  be  divided  into  two  general  groups: 
continuous  and  discontinuous.  Properties  that  can  be  repre- 
sented by  a  smooth  curve  such  as  the  curve  of  hardness  (Fig.  299), 
are  continuous  vectorial  properties,  and  those  that  vary  discon- 
tinuously  such  as  etch-figures  (Fig.  300)  are  discontinuous  vec- 
torial properties.  The  following  is  a  tabulation  of  some  of  the 
prominent  physical  properties. 

Scalar  Properties 

Specific  Gravity 
Specific  Heat 
Vectorial  Properties 

Continuous 
Hardness 
Elasticity 
Optical,  p.  156 
Ellipsoidal  I  Thermal 
Properties     Magnetic 
Electr  c 

Discontinuous 
Cleavage,  p.  129 
Etching,  p.  66 

The  discontinuous  vectorial  properties,  cleavage  and  etching, 
are  so  closely  related  to  the  morphological  properties  that  they 
have  been  described  under  that  heading  rather  than  under  the 
physical  properties. 

Of  all  the  continuous  vectorial  properties  the  optical  properties 

147 


148        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


are  the  most  important  from  both  the  theoretical  and  practical 
standpoints  and  for  that  reason  they  are  treated  in  a  separate 
division. 


Fm.  299. — Curve  of  hardness  on 
cube  face  of  fluorite. 


FIG.  300. — Etch-figure  on  cube 
face  of  fluorite. 


1.  SPECIFIC  GRAVITY 

The  density  of  a  substance  compared  with  the  density  of  water 
under  standard  conditions  (4°C.)  is  called  the  specific  gravity. 
A  specific  gravity  of  3  means  that  the  substance  weighs  three 
times  as  much  as  an  equal  volume  of  water.  The  specific  gravity 
is  the  weight  of  a  substance  divided  by  the  weight  of  an  equal 
volume  of  water.  Five  methods  of  finding  the  specific  gravity 
are  described. 

(a)  A  rough,  but  rapid,  method  is  to  weigh  out  a  gram  of  the 
mineral  and  then  find  its  volume  with  a  burette.  Care  must  be 
taken  to  eliminate  air-bubbles.  If  one  gram  is  used,  the  specific 
gravity  is  the  reciprocal  of  the  volume. 

(6)  A  more  accurate  method  is  based  on  the  fact  that  a  body 
immersed  in  water  loses  in  weight  an  amount  equal  to  the  weight 
of  the  water  displaced.  The  substance  has  the  weight  A  in  air, 
say.  Suspended  by  a  fine  thread  in  a  vessel  of  water,  it  has  the 

weight  W.     Then  G  =   A_W>  wnere  G  is  the  specific  gravity. 

Numerous  precautions  must  be  taken  to  insure  accuracy. 

(c)  A  convenient  specific  gravity  balance  for  the  practical 
identification  of  minerals  is  that  represented  in  Fig.  301,  which 
was  designed  by  the  author.  It  consists  essentially  of  a  brass 


THE  PHYSICAL  PROPERTIES  OF  MINERALS 


149 


beam  supported  near  one  end  by  a  knife  edge.  The  short  arm 
carries  two  pans;  the  lower  one  is  immersed  in  water.  The 
end  of  the  long  arm  rests  within  a  guard,  which  limits  the  motion 
of  the  balance.  The  long  arm  of  the  beam  is  graduated  so  that 
the  specific  gravity  may  be  read  off  directly.  This  may  be 
done  by  always  placing  the  counterpoise  in  the  notch  near  the 
end  of  the  long  arm  when  weighing  in  air.  Whatever  the  weight 
of  the  counterpoise,  its  distance  (x)  from  the  fulcrum  and  the 
distance  (y)  of  the  counterpoise  from  the  fulcrum  when  weighing 

T* 

in  water  are  connected  by  the  equation,  G  =—    — ,  G  being  the 

x  —  y 

specific  gravity.     The  distance  y  for  various  values  of  G  is  de- 


FIG.  301. — Specific  gravity  balance. 

termined  and  the  corresponding  value  for  G  is  marked  on  the 
beam.  Thus  in  the  balance  figured,  x  is  15  inches.  Then  if  G  is 
2,  y  is  7.5;  so  2  is  marked  at  a  point  7.5  inches  from  the  fulcrum. 
If  G  is  3,  y  is  10;  so  3  is  marked  at  a  point  10  inches  from 
the  fulcrum.  The  balance  is  adjusted  by  a  device  just  above  the 
fulcrum.  When  in  adjustment  the  balance  will  look  like  the 
figure,  the  lower  pan  being  immersed  in  water  and  the  long  arm 
of  the  balance  free.  The  mineral  is  placed  on  the  upper  pan 
and  the  counterpoise  in  the  notch  near  the  end  of  the  long  arm. 
Wire  loops  are  added  to  the  hook  of  the  counterpoise  until  the 
mineral  is  balanced.  Then  the  mineral  is  transferred  to  the 
lower  pan.  (It  is  well  to  moisten  the  mineral  before  immersion 
so  as  to  free  it  of  air  bubbles.)  The  mineral  will  lose  weight,  so 


150        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

the  counterpoise  is  moved  toward  the  fulcrum  until  balance  is 
restored.  The  specific  gravity  is  indicated  directly  on  the  beam. 
This  method  is  rapid,  and  the  results  are  accurate  enough  for  the 
practical  purposes  of  determination. 

(d)  For  accurate  work  a  pycnometer  or  specific  gravity  flask 
may  be  used.     The  pycnometer  itself  is  first  weighed  (A).     The 
coarsely  powdered  mineral  is  introduced  into  the  pycnometer 
and  another  weighing  (B)  made.     The  flask  is  then  filled  with 
distilled  water,  and  air  bubbles  are  eliminated  by  boiling.     After 
cooling,  the  weight   (C)   is  taken.     Then  the  flask  is  emptied 

and     filled     with     distilled     water,     weight     (D).     Then  G  = 
£ ^ 

n  -L.  #  _  A    _  rf     With  proper  precautions  this  method  is  very 

accurate.  Fibrous  or  porous  minerals  should  be  finely  powdered, 
otherwise  the  value  is  too  low. 

(e)  A  number  of  heavy  liquids  are  useful  in  determining  the 
specific  gravity.     Methylene  iodid  (CH2I2)  has  a  specific  gravity 
of  3.3  and  may  be  diluted  with  benzol  (sp.  gr.  =  0.98) ;  this  forms 
a  liquid  with  any  desired  intermediate  specific  gravity.     A  water 
solution  of  potassium  mercuric  iodid  (KI.HgI2),  also  called  Thou- 
le"t  solution,  has  a  specific  gravity  of  3.19  and  may  be  mixed  with 
water  in  any  proportion.     The  specific  gravity  of  a  mineral  may 
be  determined  by  diluting  these  liquids  until  fragments  of  the 
mineral  neither  sink  nor  float,  but  remain  suspended.     A  West- 
phal  balance  is  then  used  to  determine  the  specific  gravity  of  the 
liquid.     The  heavy  liquids  are  especially  useful  in  separating 
mixtures  of  minerals  for  the  purpose  of  analysis. 

The  specific  gravities  of  the  common  and  important  minerals 
are  given  below. 

MINERALS  ARRANGED  ACCORDING  TO  SPECIFIC  GRAVITY 

1.0  1.7  Sylvite 

Ice  1.8  2.1 

1.6  1.9  Chabazite 

Carnallite  2.0  CHRYSOCOLLA 

Ulexite  SULFUR  GRAPHITE 


THE  PHYSICAL  PROPERTIES  OF  MINERALS 


151 


HALITE 

Scapolite 

APATITE 

Hy  d  rom  agnesite 

Vivianite 

Diopside 

Kainite 

2.7 

FLUORITE 

OPAL 

Andesine 

Forsterite 

Stilbite 

Anorthite 

Jarosite 

2.2 

Beryl 

HORNBLENDE 

Analcite 

Bytownite 

PYROXENE 

Chalcanthite 

CALCITE 

Sillimanite 

Chrysotile 

Labradorite 

3.3 

Cristobalite 

PLAGIOCLASE 

Augite 

Halloysite 

Scapolite 

Axinite 

Heulandite 

TALC 

Clinozoisite 

Natrolite 

Turquois 

Enstatite 

2.3 

2.8 

OLIVINE 

Apophyllite 

CHLORITE 

3.4 

Glauconite 

COLLOPHANE 

CALAMINE 

Tridymite 

DOLOMITE 

EPIDOTE 

GYPSUM 

Lepidolite 

Hypersthene 

Nitratine 

MUSCOVITE 

Vesuvianite 

Sodalite 

Pyrophyllite 

3.5 

2.4 

Phlogopite 

Diamond 

Brucite 

Sericite 

Rhodochrosite 

Golem  anite 

Wollastonite 

Titanite 

Gibbsite 

2.9 

Topaz 

Lazurite 

ANHYDRITE 

3.6 

2.5 

Aragonite 

GARNET 

ANTIGORITE 

BIOTITE 

Kyanite 

CLIACHITE 

Datolite 

Rhodonite 

Garnierite 

Prehnite 

Spinel 

Leucite 

3.0 

3.7 

2.6 

Cryolite 

Staurolite 

Adularia 

Dahllite 

Strontianite 

Albite 

Tremolite 

3.8 

Alunite 

3.1 

Azurite 

CHALCEDONY 

Andalusite 

GARNET 

Kaolinite 

Anthophyllite 

LIMONITE 

Microcline 

Chondrodite 

PSILOMELANE 

ORTHOCLASE 

Glaucophane 

SIDERITE 

Nepheline 

Magnesite 

3.9 

Oligoclase 

Spodumene 

Brochantite 

QUARTZ 

TOURMALINE 

Celestite 

PLAGIOCLASE 

3.2 

MALACHITE 

152        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


4.0 

Pentlandite 

Anglesite 

CORUNDUM 

Pyrolusite 

6.4 

GARNET 

4.9 

Bismuthinite 

PSILOMELANE 

Marcasite 

6.5 

SPHALERITE 

5.0 

CERUSSITE 

4.1 

PYRITE 

6.6 

Carnotite 

5.1 

6.7 

Turyite 

BORNITE 

Wulfenite 

Willemite 

Franklinite 

6.8 

4.2 

MAGNETITE 

Pyromorphite 

CHALCOPYRITE 

5.2 

Vanadinite 

PSILOMELANE 

HEMATITE 

6.9 

Rutile 

Stibiconite 

7.0 

4.3 

5.3 

CASSITERITE 

Goethite 

5.4 

Pitchblende 

Manganite 

5.5 

7.1 

Turyite 

Cerargyrite 

7.2 

Witherite 

5.6 

Mimetite 

4.4 

Columbite 

7.3 

CHROMITE 

5.7 

Argentite 

Enargite 

CHALCOC1TE 

7.4 

SMITHSONITE 

Jamesonite 

Wolframite 

4.5 

5.8 

7.5 

BARITE 

Pyrargyrite 

GALENA 

STIBNITE 

5.9 

Iron 

Turyite 

Columbite 

Pitchblende 

4.6 

6.0 

8.0 

Covellite 

ARSENOPYRITE 

CINNABAR 

PYRRHOTITE 

Cuprite 

Pitchblende 

Zircon 

Scheelite 

8.8 

4.7 

6.1 

Calaverite 

Ilmenite 

Polybasite 

COPPER 

Molybdenite 

6.2 

10.5 

TETRAHEDRITE 

Columbite 

SILVER 

Turyite 

Smaltite 

15  to  19 

4.8 

Stephanite 

GOLD 

Hausmannite 

6.3 

Platinum 

2.  HARDNESS 

The  resistance  that  a  substance  offers  to  abrasion  is  called 
hardness.     It  is  not  a  property  that  is  capable  of  exact  definition 


THE  PHYSICAL  PROPERTIES  OF  MINERALS  153 

or  measurement,  but  comparative  tests  are  expressed  in  terms  of 
a  so-called  scale  of  hardness.  The  scale  of  hardness  consists  of 
ten  minerals  ranging  from  talc,  a  mineral  which  has  a  soapy  feel 
and  is  very  easily  scratched  by  the  finger  nail,  up  to  diamond,  the 
hardest  known  substance.  The  scale  of  hardness  is  as  follows: 

Scale  of  Hardness 

1  Talc 

2  Gypsum 
Finger  Nail 

3  Calcite 

4  Fluorite 

5  Apatite 
Knife  Blade 

6  Orthoclase 

7  Quartz 

8  Topaz 

9  Corundum 
10  Diamond 


The  finger  nail  is  about  2J£,  for  it  scratches  gypsum,  but  is 
scratched  by  calcite.  A  knife  blade  is  about  5}^,  for  it  scratches 
apatite,  but  is  scratched  by  orthoclase.  The  hardness  of  a 
mineral  is  judged  both  by  its  effect  on  the  minerals  of  the  scale 
and  their  effect  upon  it.  If  a  mineral  scratches  fluorite  but  is 
scratched  by  apatite,  it  has  a  hardness  of  4J^.  Two  minerals  of 
the  same  hardness  will  scratch  each  other. 

Great  care  should  be  used  in  determining  the  hardness.  A 
foreign  substance  embedded  in  the  mineral  will  often  give  too 
high  a  value.  A  soft  mineral  leaves  a  "  chalk-mark"  on  a  harder 
one,  so  the  mark  left  by  a  mineral  should  be  a  distinct  groove. 
Minerals  made  up  of  grains  or  fibers  often  appear  too  low  simply 
because  the  particles  are  forced  apart.  Thus  a  sandstone  made 
up  of  sand  grains  with  hardness  of  7  may  appear  to  have  a  hard- 
ness of  about  3  simply  because  the  grains  are  loosely  cemented. 
The  value  recorded  in  the  description  of  minerals  is  the  maximum 
value  for  well-crystallized  varieties. 


154        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

In  crystals  the  hardness  varies  with  the  direction,  as  do  prac- 
tically all  the  physical  properties  except  specific  gravity.  This 
is  shown  in  Fig.  299,  p.  148.  The  amount  of  abrasion  of  the 
crystal  is  determined  for  a  number  of  directions  by  mounting 
the  crystal  on  an  instrument  known  as  a  sclerometer.  The 
values  are  plotted  and  connected  by  a  smooth  curve  known  as 
the  curve  of  hardness.  The  most  remarkable  case  of  variation 
of  the  hardness  with  direction  is  probably  that  of  kyanite  (tri- 
clinic  A^SiOs).  Kyanite  in  a  direction  parallel  to  the  c-axis 
has  a  hardness  of  4J^,  while  at  right  angles  to  this  the  hardness 
is  about  7.  For  most  faces  calcite  has  a  hardness  of  3,  but  on 
the  basal  pinacoid  (0001)  the  hardness  is  about  2,  as  this  face  is 
easily  scratched  by  the  finger  nail. 

3.  LUSTER 

Luster  is  the  term  applied  to  the  quality  of  light  reflected  from 
a  substance.  Metallic  luster  is  the  brilliant  luster  of  metals 
possessed  by  most  sulfid  minerals  such  as  galena,  pyrite,  etc.,  as 
well  as  some  oxids  such  as  hematite  and  magnetite.  Minerals 
with  metallic  luster  are  opaque  even  on  the  thinnest  edges. 

Adamantine  is  the  brilliant  luster  of  transparent  or  translucent 
minerals  with  high  index  of  refraction.  Examples  are  diamond 
(n  =  2.41)  and  cerussite  (n  =  1.80-2.07).  Vitreous  is  the 
luster  of  broken  glass  possessed  by  most  transparent  or  trans- 
lucent minerals  such  as  quartz,  calcite,  etc.  Pearly  luster  is 
due  to  continued  reflection  from  a  series  of  parallel  plates,  and  is 
possessed  by  minerals  with  eminent  cleavage  such  as  gypsum  and 
talc.  Silky  luster  is  due  to  fibrous  structure  and  is  illustrated  by 
chrysotile  and  fibrous  gypsum.  Waxy,  greasy,  pitchy,  and  dull 
are  self-explanatory  terms  used  to  describe  luster. 

4.  COLOR 

The  term  color  is  a  general  one,  and  for  convenience  usually 
includes  black  (which  really  means  the  absence  of  color)  and  white 
(the  union  of  all  colors).  The  solar  spectrum  is  the  standard  for 


THE  PHYSICAL  PROPERTIES  OF  MINERALS  155 

non-metallic  colors.  The  term  hue  refers  to  a  particular  part  of 
the  spectrum.  Thus  we  speak  of  an  orange  hue  or  a  green 
hue.  A  given  spectrum  hue  illuminated  (or  mixed  with  white) 
becomes  a  tint,  while  a  given  hue  with  insufficient  illumination 
(or  mixed  with  black)  becomes  a  shade.  Thus  pink  is  a  tint  of 
red,  and  olive  green  a  shade  of  yellow.  Besides  tints  and  shades 
produced  from  the  spectrum  we  have  other  colors  formed  by 
mixing  the  spectrum  hues  with  various  grays  (mixtures  of  black 
and  white). 

In  some  minerals  such  as  cinnabar,  orpiment,  malachite,  and 
azurite  the  color  is  a  property  of  the  substance  and  hence  is 
constant.  The  color  of  a  metallic  mineral  is  usually  quite  con- 
stant, but  as  these  minerals  are  susceptible  to  tarnish  a  fresh 
fracture  should  always  be  observed. 

But  in  the  majority  of  non-metallic  minerals  the  color  is  due 
to  some  impurity  which  usually  exists  in  very  small  amounts, 
and  is  often  present  as  a  pigment  in  solid  solution.  Thus  quartz, 
calcite,  and  fluorite  are  colorless  when  pure,  but  they  are  found  in 
various  tints  and  shades  of  practically  all  hues  which  may  vary 
even  in  the  same  specimen. 

6.  STREAK 

The  streak  of  a  mineral  refers  to  the  color  of  its  powder. 
It  may  be  determined  by  rubbing  a  corner  of  the  mineral  on  a 
piece  of  unglazed  porcelain  or  tile  called  a  streak-plate.  In 
the  absence  of  a  streak-plate  a  smooth  piece  of  light  colored  flint 
or  chert  may  be  used.  A  thin  slab  of  novaculite  also  makes  an 
excellent  streak-plate. 

The  streak,  though  colorless  for  most  non-metallic  minerals 
and  dark-gray  or  black  for  many  metallic  ones,  is  especially 
valuable  in  the  determination  of  a  few  common  minerals  such  as 
hematite  (streak,  red-brown)  and  limonite  (streak,  yellow-brown) . 


THE  OPTICAL  PROPERTIES  OF  MINERALS 

Among  the  physical  properties,  the  optical  properties  take  first 
rank  in  the  accurate  description  and  determination  of  all  minerals 
that  transmit  light  in  thin  layers,  for  the  mineral  may  be  deter- 
mined even  in  the  absence  of  distinct  crystals  and  when  occurring 
in  small  quantities. 

Most  of  the  optical  determinations  can  be  made  by  means  of  a 
special  form  of  microscope  known  as  the  polarizing  microscope, 
but  for  the  more  accurate  determination  of  the  optical  constants 
the  refractometer,  the  goniometer,  and  the  axial  angle  apparatus 
must  be  used. 

Minerals  for  optical  determinations  may  be  prepared  in  three 
different  forms:  (1)  oriented  sections  or  sections  cut  in  definite 
crystallographic  directions,  (2)  thin  sections  in  which  the  con- 
stituent minerals  are  cut  at  random  (by  this  means  minerals 
in  fine  grained  rocks  may  be  determined),  and  (3)  fragments, 
cleavage  flakes,  and  minute  crystals.  As  fragments  are  easily 
prepared  simply  by  crushing  the  mineral,  the  method  is  a  general 
one  for  the  examination  of  all  but  opaque  minerals.  The  polariz- 
ing microscope  should  have  a  place  in  the  mineralogical  laboratory 
and  should  supplement  the  blowpipe  in  the  determination  of  minerals. 
It  is  safe  to  say  that,  generally  speaking,  it  is  impossible  to  deter- 
mine the  less  common  non-metallic  minerals  without  the  use  of 
the  polarizing  microscope. 

1.  THE  NATURE  OF  LIGHT 

It  is  now  generally  believed  that  light  consists  of  a  vibratory 
motion  or  some  kind  of  disturbance  in  the  ether,  a  hypothetical 
medium  which  is  supposed  to  pervade  all  space  and  even  material 
bodies.1  The  wave-motion,  as  it  is  called,  is  regarded  as  the 

1  Light  is  a  form  of  energy  and  as  we  pan  not  conceive  of  energy  being  transmitted  without 
Borne  kind  of  medium,  physicists  postulate  a  something  different  from  ordinary  matter 
which  they  call  the  ether. 

156 


THE  OPTICAL  PROPERTIES  OF  MINERALS 


157 


resultant  of  simple  harmonic  motion  and  a  uniform  linear  motion 
at  right  angles  to  this.  Simple  harmonic  motion  is  uniform 
motion  in  a  circular  path  as  it  would  appear  on  a  diameter  of  the 
circle.  The  circle  of  reference,  as  it  is  called,  is  shown  in  Fig.  302 
(adgj).  A  point  moving  from  a  to  d  appears  to  move  from  a'  to 
d.  This  constitutes  a  periodic  vibration  with  varying  velocity 
which  may  be  represented  by  the  swinging  of  a  pendulum.  If 
this  is  compounded  with  linear  motion  from  A  to  A',  we  have  the 
harmonic  curve  ADGJA',  which  is  a  sine  curve  represented 

by  the  equation  y  =  a  sin  -,x,  in  which  a  is  OD,  the  amplitude 


j/_X 

6'   /\f 

x^ 

c 

J    E 

\ 

u3 

g3 

z 

B 

O 

F 

\ 

\^7 

V-w 

A 

G 

\ 

/ 

\z 

A;  "  —  ! 

V 

H 

X 

I 

T     K 

/ 

L 

FIG.  302. — Wave-motion. 


and  JJT,  the  angular  velocity  in  the  circle. 


Fig.  302  illustrates 


the  wave-motion  at  a  given  instant. 

Thus  light  consists,  to  the  best  of  our  knowledge,  of  a  periodic 
vibration  transverse  to  the  direction  of  transmission,  though  we 
know  nothing  of  its  physical  nature.  It  may  be  illustrated  in  a 
rough  way  by  the  waves  observed  along  the  sea-shore.  A  float- 
ing object,  in  general,  simply  moves  up  and  down,  while  the  wave 
as  a  whole  advances  toward  the  shore. 

The  maximum  displacement,  OD  (Fig.  302),  is  called  the 
amplitude  of  the  vibration.  The  period  is  the  interval  of  time 
necessary  for  a  complete  vibration.  The  point  with  the  maximum 
upward  displacement,  D,  is  called  the  crest  of  the  wave  and  the 
point  with  maximum  downward  displacement,  J,  the  trough. 


158        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

The  distance  between  two  successive  crests  or  troughs,  or  a 
corresponding  distance  such  as  AA',  is  called  the  wave-length 
(denoted'  by  the  Greek  letter  \  [lambda]) .  The  number  of  vibra- 
tions executed  in  a  unit  of  time  defines  the  frequency. 

By  phase  is  meant  the  relative  position  of  any  two  points. 
Two  points  such  as  A  and  A'  are  in  the  same  phase  when  they 
are  in  the  same  relative  position  and  moving  in  the  same  direction. 
Two  points  such  as  A  and  G  are  said  to  be  in  opposite  phase  when 
they  are  in  the  same  relative  position  but  moving  in  opposite 
directions. 

A  ray  of  light  is  a  line  used  to  indicate  the  direction  of  trans- 
mission of  the  wave-motion,  but  in  practice  beams  of  light  must 
be  employed.  A  wave-front  is  the  surface  determined  by  all  the 
parts  of  a  system  of  waves  which  are  in  the  same  phase.  A  ray 
is  perpendicular  to  its  wave-front  in  an  optically  isotropic 
medium. 

The  intensity  of  light  depends  upon  the  amplitude  of  the  vibra- 
tions, and  the  color  of  the  light  depends  upon  the  wave-length  of 
the  vibrations  or,  more  accurately,  upon  the  frequency,  for  the 
wave-length  is  altered  when  the  light  enters  any  material  medium. 
The  wave-length  for  the  violet  end  of  the  spectrum  is  380/x/* 
(millimicrons  or  millionths  of  a  millimeter)  and  for  the  red  end  of 
the  spectrum,  760/-IM  (millionths  of  a  millimeter).  White  light 
is  the  sum  of  light  of  the  various  waves  which  together  produce 
the  solar  spectrum.  For  this  reason  monochromatic  light  (light 
of  approximately  one  wave-length)  must  be  employed  in  all 
accurate  optical  work.  The  simplest  method  of  obtaining  mono- 
chromatic light  is  to  ignite  a  sodium  salt  on  platinum  wire  in  a 
dark  room.  Yellow  light  with  a  wave-length  of  589^  is  pro- 
duced. For  red  light  a  lithium  salt  is  used  and  for  green  light  a 
thallium  salt.  In  the  laboratory  it  is  more  convenient  to  use  a 
colored  screen  which  gives  light  with  a  considerable  range  of 
wave-lengths,  but  for  quantitative  work  strictly  monochromatic 
light  must  be  used. 


THE  OPTICAL  PROPERTIES  OF  MINERALS 


159 


2.  REFRACTION  OF  LIGHT 

When  a  beam  of  light  passes  from  one  medium  into  another,  in 
general  there  is  a  change  of  direction,  which  is  due  to  the  fact 
that  there  is  a  change  of  velocity  in  the  light  waves.  This  is 
shown  in  Fig.  303.  Parallel  rays  pp'  on  passing  from  air,  say, 
into  a  section,  arrive  at  the  points  rr'  together.  But  while  the 
impulse  along  the  ray  p'  is  going  the  distance  rV  in  the  air,  that 
along  the  ray  p  has  gone  the  distance  rs,  the  velocity  in  the  sec- 
tion being  less  than  in  air.  The  wave-front  in  the  section  is 
then  ss'  constructed  by  drawing  a  line  from  sf  tangent  to  an  arc 
with  radius  rs  (rs  being  the  velocity 
in  the  action  as  compared  with  velo- 
city r's'  in  air).  The  beam  of  light  is 
bent  toward  the  perpendicular.  This 
phenomenon  is  known  as  refraction. 
A  familiar  illustration  is  the  apparent 
bending  of  a  stick  in  water.  In  Fig. 
304  the  angle  i  is  called  the  angle  of 
incidence,  and  the  angle  r,  the  angle  of 
refraction.  The  radii  of  the  two  con- 
centric circles  are  proportional  to  the 
indices  of  refraction  of  the  two  sub- 
stances (air  and  a  liquid  in  this  case).  The  value  of  r  for  any 
given  value  of  i  may  be  found  by  extending  the  incident  ray 
until  it  cuts  the  smaller  concentric  circle.  From  this  intersection 
a  perpendicular  is  dropped  to  the  bounding  surface  of  the  two 
media.  A  line  drawn  from  the  intersection  of  this  perpendicular 
with  the  larger  circle  to  the  center  gives  the  direction  of  the 
refracted  ray.  There  is  found  to  be  a  constant  relation  between 
the  sines  of  these  angles,  for  whatever  the  direction  of  transmis- 
sion, -  -  =  n  (a  constant).  The  constants,  which  is  called  the 
sin  r 

index  of  refraction,  depends  upon  the  substance  and  upon  the 
kind  of  monochromatic  light  used.  The  index  of  refraction  for 
the  violet  end  of  the  spectrum  is  greater  than  for  the  red  end  of 


FIG.    303. — The      relation      of 
velocity  to  index  of  refraction. 


160        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


the  spectrum  as  shown  in  Fig.  305.  For  diamond  the  dispersion 
or  difference  between  the  values  of  the  refractive  indices  for 
opposite  ends  of  the  spectrum  is  very  large  (nv  —  nr  =  0.063), 
hence  the  "  fire  "  of  the  diamond.  For  fluorite,  on  the  other  hand, 
the  dispersion  is  very  small  (nv  —  nr  =  0.006),  hence  fluorite 


FIG.  304. 


FIG.  305. 


is  often  used  in  making  achromatic  microscope  lenses.  The  follow- 
ing list  gives  the  indices  of  refraction  (yellow  light)  for  some  of  the 
common  minerals: 

Fluorite 

Gypsum 

Quartz 

Barite 

Apatite 

Corundum 


Cerussite 
Sphalerite 


1.434 

1.520-1.529 
1.544-1.553 
1.636-1.647 
1 . 635-1 . 638 
1.759-1.767 
1.804-2  ..078 
2.369 


The  minerals  in  the  above  list  with  the  exception  of  fluorite 
and  sphalerite  are  doubly  refracting  and  so  the  index  of  refrac- 
tion ranges  from  a  minimum  to  a  maximum  depending  upon  the 
direction  of  the  light  rays. 

Substances  with  a  high  index  of  refraction  (1.9  or  over)  have 
the    brilliant    appearance    called    adamantine    luster,    minerals 
with  an  index  of  refraction  lower  than  1.7  have  ordinary  vitreous 
luster,  while  those  between  1.7   and  1.9  (e.g.  corundum)  have   i 
subadamantine  luster. 


THE  OPTICAL  PROPERTIES  OF  MINERALS  161 

Several  particular  directions  of  transmission  should  be  men- 
tioned.    In  the  formula  -  -  =  n,  if  i  =  0°,  r  =0°;  so  for  normal 
sin  T 

incidence  there  is  no  refraction.  If  i  —  90°,  the  equation  becomes 
sin  r  =  -;  r  for  this  particular  value  is  called  the  critical  angle, 

The  critical  angle  like  the  index  of  refraction  is  a  constant  for  the 
substance.  A  graphic  determination  of  this  angle  is  shown  in 
Fig.  306.  The  indices  of  refraction  of  the  two  substances  are 
the  radii  of  two  concentric  circles,  AB  being  the  boundary  between 
the  two  substances.  A  tangent  is  dropped  from  the  intersection 
of  the  inner  circle  with  the  boundary  line.  A  radius  is  then 
drawn  through  the  point  where  this  tangent  intersects  the  outer 
circle.  The  angle  PON  is  the  critical  angle. 

Rays  of  light  passing  from  the  denser  (lower)  medium  to  the 
rarer  (upper)  medium  along  the  line  PO  will  graze  the  surface 
OA.  Rays  of  light  passing  from  the  denser  into  the  rarer  medium 
at  angles  greater  than  the  critical  angle  cannot  enter  the  rarer 
medium,  but  are  reflected  back  into  the  denser  medium  as  illus- 
trated by  the  dotted  line  of  Fig.  306.  This  phenomenon  is  called 
total  reflection.  An  empty  test-tube  immersed  in  a  beaker  of 
water  has  a  peculiar  silvery  appearance  caused  by  total  reflection. 
This  silvery  reflection  disappears  when  the  test-tube  is  filled  with 
water.  The  reason  for  the  silvery  appearance  may  be  shown  by 
constructing  the  critical  angle  for  water  and  air  on  a  drawing  of 
the  test-tube  in  a  beaker  of  water. 

The  great  brilliancy  of  the  cut  diamond,  as  compared  with 
natural  crystals  of  the  uncut  diamond,  is  due  principally  to  the 
fact  that  the  facets  are  arranged  so  that  most  of  the  light  is 
totally  reflected,  for  the  critical  angle  for  diamond  is  very  small 
(24°  as  compared  with  48°  for  ordinary  glass). 

Direct  Determination  of  the  Index  of  Refraction 

The  index  of  refraction  is  the  fundamental  optical  constant,  and 

its  determination  is  one  of  the  best  means  of  identifying  a  given 
n 


162        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

mineral  or  other  substance,  provided  of  course  that  it  transmits 
light. 

The  most  direct  procedure  for  determining  the  index  of  re- 
fraction is  called  the  prism  method.  The  crystal  cut  in  the  form 
of  a  prism  of  about  60°  (internal  angle)  is  mounted  on  a  reflec- 
tion goniometer  or  spectrometer.  A  narrow  beam  of  light  from 
the  collimator  is  refracted  both  on  entering  and  emerging  from 
the  prism,  as  shown  diagrammatically  in  Fig.  307.  If  the  prism 

C 


N  M 

FIG.  307. — Determination  of  index  of  refraction  by  the  prism  method. 

were  not  present  the  beam  of  light,  CO,  would  reach  the  point 
M  but  instead  it  is  deviated  out  of  its  course  and  reaches  a  point 
such  as  N.  The  direct  reading  with  the  telescope  at  M  is  taken 
and  then  the  telescope  is  moved  to  the  left.  The  crystal  is  then 
revolved  on  the  axis  of  the  goniometer  until  the  refracted  image 
is  in  the  field  of  view.  The  crystal  and  telescope  are  then  manipu- 
lated, one  with  each  hand,  until  the  image  first  moves  in  one  direc- 
tion and  directly  afterwards  in  the  opposite  direction.  This 
momentary  and  stationary  position  of  the  image  determines  the 


THE  OPTICAL  PROPERTIES  OF  MINERALS 


163 


angle  of  minimum  deviation,  d,  from  which  the  index  of  refrac- 
tion may  be  calculated  by  the  equation  n  =  -  —    ~    — 


sin 

where  p  is  the  internal  angle  of  the  prism.  In  doubly  refracting 
crystals  two  values  of  the  index  of  refraction  will  in  general  be 
obtained.  The  index  of  refraction  of  a  liquid  may  be  determined 
by  placing  it  in  a  hollow  prism  and  proceeding  as  above. 

Other  methods  of  determining  the  index  of  refraction  depend 
upon  finding  the  value  of  the  critical  angle.  For  this  purpose 
an  instrument  known  as  a  refractometer  is  used.  Now  the  criti- 


Fio.  308. — Determination  of  the  index 
of  refraction  by  total  reflection. 


FIG.  309. — Determination  of  the 
index  of  refraction  by  grazing 
incidence. 


cal   angle  may  be  determined  in   two   different  ways:  (1)  by 
total  reflection  proper  and  (2)  by  grazing  incidence. 

The  first  method  is  shown  in  Fig.  308.  The  crystal  or  vessel 
with  enclosed  liquid  is  placed  on  the  upper  plane  surface  of 
highly  refracting  glass  (a  necessary  condition  is  that  the  glass 
must  have  a  higher  index  of  refraction  than  the  substance  to 
be  tested)  and  diffused  light  is  directed  upward  on  one  side  of 
the  glass  hemisphere.  On  striking  a  substance  with  lower  index 
of  refraction  some  of  the  rays  will  enter  the  substance  but  those 
that  meet  it  at  an  angle  greater  than  the  critical  angle  will  be  re- 
flected back  into  the  hemisphere.  This  will  cause  half  of  the  field 
of  the  telescope  to  be  partially  dark  and  the  other  half  light  as 


164        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

shown  in  Fig.  308.  If  the  sharp  line  of  demarcation  is  placed  on 
the  cross  hairs  of  the  telescope,  the  critical  angle  may  be  read  off. 
directly. 

In  the  other  method,  that  of  grazing  incidence,  the  light  enters 
the  crystal,  or  liquid  in  the  vessel,  from  the  side,  as  shown  in  Fig. 
309.  In  this  case  the  upper  side  of  the  telescope  field  is  dark, 
for  no  totally  reflected  light  is  allowed  to  fall  upon  the  lower 
part  of  the  hemisphere. 

In  the  Abbe*  refractometer  the  principle  of  total  reflection 
proper  is  used,  while  in  the  Pulfrich  refractometer  the  method 
of  grazing  incidence  is  employed.  Either  of  these  methods  may 
be  used  for  solids  or  liquids. 


FIG.  310. — The  Smith  refractometer. 

A  convenient  refractometer  for  the  approximate  determination 
of  the  refractive  indices,  especially  for  cut  gem  stones,  is  one 
devised  by  G.  F.  H.  Smith  of  the  British  Museum.  This  refracto- 
meter is  shown  diagrammatically  in  cross-section  in  Fig.  310. 
It  consists  of  a  metal  frame  holding  a  hemisphere,  h,  of  highly 
refracting  glass  (n  =  1.79),  a  totally-reflecting  prism, P,  and  lenses 
at  V,  I2,  and  Is.  The  substance,  c,  the  index  of  refraction  of 
which  is  sought,  is  placed  over  the  glass-hemisphere,  h  and  in  close 
contact  with  it  by  means  of  a  drop  of  a-monobromnaphthalenc 
or  methylene  iodid.  As  the  glass  hemisphere  has  a  greater  index 
of  refraction  than  the  substance  (this  is  a  necessary  condition), 
rays  of  light  entering  at  V  are  in  part  totally  reflected  back  into 


THE  OPTICAL  PROPERTIES  OF  MINERALS  165 

the  hemisphere,  the  rays  represented  by  the  dotted  lines  entering 
the  substance.  The  totally  reflected  rays  fall  on  a  scale,  S,  en- 
graved on  glass  and  are  reflected  by  the  prism  P  into  the  eye-piece 
at  P.  The  field  of  vision  appears  as  in  the  circle  above  the  eye- 
piece; one-half  of  it  is  light  and  the  other  half  dark.  After  the 
instrument  has  been  calibrated  the  index  of  refraction  may  be 
read  off  directly  on  the  scale.  The  reading  in  the  figure  indicates 
a  doubly  refracting  substance  with  indices  of  refraction  of  1.559 
and  1.588.  Accurate  observations  should  be  made  in  mono- 
chromatic light,  but  examination  in  white  light  will  indicate  the 
amount  of  dispersion. 

Indirect  Determination  of  the  Index  of  Refraction 

A  simple,  but  indirect,  method  of  determining  the  index 
of  refraction  of  a  mineral  is  by  means  of  the  Becke  test.  Frag- 
ments of  the  mineral  are  embedded  in  a  liquid  of  known  index 
of  refraction  and  examined  on  the  stage  of  a  microscope 
with  the  diaphragm  of  the  substage  partially  closed.  (In  the 
absence  of  a  diaphragm  the  substage  should  be  lowered.)  On 
focusing  sharply  on  the  edge  of  the  mineral  and  then  throwing  it 
slightly  out  of  focus  by  raising  the  microscope  tube,  a  blurred 
white  line  will  appear  on  the  side  of  the  substance  having  the 
greater  index  of  refraction.  If  the  fragment  is  small  enough, 
the  fragment  as  a  whole,  not  simply  the  border,  will  become 
brighter  if  its  index  is  greater  than  that  of  the  liquid  and  darker 
if  the  index  is  less  than  that  the  liquid.  On  lowering  the  microscope 
tube  the  white  line  appears  on  the  side  of  the  substance  having 
the  smaller  index  of  refraction.  The  explanation  of  the  Becke 
test  is  given  in  Figs.  311-312.  Two  fragments  are  shown  em- 
bedded in  a  liquid.  The  fragment  on  the  left  has  an  index  of 
refraction  greater  than  that  of  the  liquid,  while  with  the  one  on 
the  right  the  opposite  is  the  case.  The  microscope  tube  is 
supposed  to  be  raised.  In  the  first  case  the  rays  of  light  on 
striking  the  oblique  boundary  are  reflected  back  into  the  mineral, 
for  the  critical  angle  is  exceeded.  In  the  other  case  the  rays 


166        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


go  on  through  the  mineral  because  the  critical  angle  is  not  ex- 
ceeded. If  then  the  focus  is  changed  to  the  dotted  line  shown, 
it  is  apparent  that  there  will  be  a  concentration  of  light  toward 
the  fragment  in  the  first  case,  but  not  in  the  second.  By  using 


FIG.  311.  FIG.  312. 

FIGS.  311-312. — Explanation  of  the  Becke  test. 

a  number  of  liquids   the  index  may  be  obtained  within  certain 
limits.     The  most  useful  liquids  are  as  follows : 

Petroleum  .  450  ± 

Turpentine  .  472  ± 

Clove  Oil  .  530  ± 

a-monobromnaphthalene  .  658  + 

Methylene  Iodide  .  742  ± 

These  liquids  may  be  mixed  with  each  other  to  form  liquids 
of  intermediate  indices  of  refraction.  The  index  of  refraction 
of  the  liquid  must  be  determined  on  a  refractometer.  The 
Abbe*  refractometer  is  especially  convenient  because  only  one  or 
two  drops  of  the  liquid  are  necessary  and  the  index  may  be  read 
off  directly.  It  is  convenient  to  have  a  series  of  30  liquids  ranging 
from  1.45  to  1.74  and  differing  from  each  other  by  0.01.  These 
liquids  should  be  kept  in  bottles  with  double  ground  glass  stoppers 
so  that  the  index  of  refraction  will  remain  constant. 

Some  idea  of  the  index  of  refraction  may  also  be  judged  by  the 
appearance  of  the  fragment  in  the  liquid.  If  the  fragment  and 
the  liquid  have  about  the  same  index  of  refraction,  the  fragment 


THE  OPTICAL  PROPERTIES  OF  MINERALS  167 

will  appear  smooth  and  will  scarcely  be  visible;  it  is  said  to  have 
low  relief  (Fig.  314).  If,  on  the  other  hand,  the  indices  of  re- 
fraction of  the  two  substances  are  quite  different,  the  surface  of 
the  fragment  will  appear  rough  and  the  borders  dark.  In  this 
case  the  mineral  is  said  to  have  high  relief.  It  should  be  noted 
that  the  fragment  has  high  relief  whether  its  index  is  greater  (Fig. 
315)  or  less  (Fig.  313)  than  that  of  the  liquid. 

In  the  above  explanations  it  is  assumed  that  the  velocity  is 
the  same  in  all  directions  of  the  substance.  This  is  true  only  of 
amorphous  and  isometric  substances,  and  even  then  only  under 
normal  conditions.  Doubly  refracting  substances  are  treated 
in  a  later  section. 


FIG.  313.  FIG.  314.  FIG.  315. 

FIGS.  313-315. — High  and  low  relief  of  mineral  fragments  embedded  in  a  liquid. 

3.  POLARIZED  LIGHT 

That  the  light  transmitted  through  a  slice  of  a  colored  tourma- 
line crystal  cut  parallel  to  the  c-axis  has  acquired  peculiar  proper- 
ties may  be  seen  by  looking  at  one  tourmaline  through  another 
similar  tourmaline.  (The  little  device  known  as  "  tourmaline 
tongs"  shows  this  very  well.)  When  similar  directions  of  the 
two  tourmalines  of  the  right  depth  of  color  are  parallel,  a  maxi- 
mum amount  of  light  is  transmitted,  while  if  similar  directions 
of  the  tourmalines  are  perpendicular  no  light  at  all  is  transmitted. 
The  simplest  explanation  is  that  the  light  is  vibrating  in  one  plane 
only.  A  simple  experiment  proves  that  the  vibration  plane  is 
the  one  that  includes  the  c-axis  (the  axis  of  3-fold  symmetry). 
A  beam  of  light  from  an  arc  lamp  after  traversing  the  tourmaline 


168        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


crystal  and  a  colloidal  suspension  made  by  adding  an  alcoholic 
solution  of  resin  to  water  has  a  maximum  intensity  when  the 
plane  of  the  c-axis  is  normal  to  the  line  of  sight,  but  is  almost 
invisible  when  the  plane  of  the  c-axis  coincides  with  the  line  of 

sight. 

In  ordinary  light  the  vibra- 
tions are  in  all  planes,  while  the 
light  transmitted  by  the  tourma- 
line slice  is  in  but  one  plane. 
This  light  is  known  as  polarized 
light.  Figure  316  is  a  diagram- 
matic representation  of  ordinary 
FIG.  316.— Polarization  by  absorption,  light  and  polarized  light  as  trans- 
mitted by  tourmaline. 

The  light  reflected  from  non-metallic  surfaces  such  as  glass  or 
polished  wood  is  more  or  less  polarized.  If  the  light  reflected 
from  a  sheet  of  glass  is  examined  with  the  tourmaline,  it  will  be 
found  that  for  a  certain  angle  of  incidence  (56°  for  ordinary  glass) 
light  is  almost  extinguished  when  the  c-axis  of  the  tourmaline 


FIG.  317. — Polarization  by  reflection. 

is  parallel  to  the  plane  of  incidence,  while  a  maximum  amount 
of  light  is  transmitted  when  that  direction  is  perpendicular  to 
the  plane  of  incidence.  This  means  that  the  reflected  light  is 
partially  polarized,  and  that  the  plane  of  vibration  is  perpendi- 
cular to  the  plane  of  incidence  as  shown  in  Fig.  317. 

Another  method  of  producing  polarized  light  is  by  continued 


THE  OPTICAL  PROPERTIES  OF  MINERALS 


169 


refraction  through  a  series  of  parallel  glass  plates.  The  emerging 
light  is  partially  polarized  and  vibrates  in  the  plane  of  incidence. 
Figure  318  shows  this,  as  well  as  the  fact  that  the  reflected  light 
is  polarized  and  that  the  vibrations  are  in  a  plane  normal  to 
the  plane  of  incidence. 

The  most  practical  method  of  obtaining  polarized  light  is  by 
means  of  a  Nicol  prism,  but  this  involves  a  discussion  of  double 
refraction  which  is  the  next  topic. 


FIG.  318. — Polarization  by  refraction. 

4.  DOUBLE  REFRACTION 

If  a  dot  is  viewed  through  a  clear  cleavage  rhombohedron  of 
calcite  (Iceland  spar),  two  images  of  the  dot  are  seen;  on 
revolution  of  the  calcite  one  dot  remains  stationary,  while  the 
other  dot  appears  to  revolve  around  the  fixed  one.  A  ray  of 
light  thus  gives  rise  to  two  rays,  the  ordinary  ray  (the  fixed  one) 
and  the  extraordinary  ray  (the  one  that  revolves).  In  Fig.  319 
the  image  of  the  ordinary  ray  is  marked  o,  and  that  of  the  extra- 
ordinary e.  This  phenomenon  is  known  as  double  refraction 
and  though  possessed  by  most  minerals,  calcite  is  practically 
the  only  transparent  mineral  in  which  is  it  marked  enough  to 
be  seen  with  the  naked  eye. 

Light  that  emerges  from  a  doubly  refracting  substance  such  as 
calcite  has  acquired  peculiar  properties,  as  may  be  demonstrated 


170        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


by  examining  this  double  image  with  a  tourmaline  section  cut 
parallel  to  the  c-axis.  When  the  c-axis  of  the  tourmaline  is 
parallel  to  the  long  diagonal  of  the  calcite  rhomb  (Fig.  320)  only 
the  image  due  to  the  ordinary  ray,  o,  is  seen,  but  when  the 


FIG.  319. 


FIG.  320. 


FIG.  321. 


FIG.  322a.  FIG.  3226. 

FIGS.  319-3226. — Double  refraction  in  calcite. 

c-axis  of  the  tourmaline  is  parallel  to  the  short  diagonal  of  the  cal- 
cite rhomb  (Fig.  321)  only  the  image  due  to  the  extraordinary 
ray,  e,  is  seen.  Hence  for  the  ordinary  ray  the  vibrations  are  in 


FIG.  323.  FIG.  324.  FIG.  325.  FIG.  326.  FIG.  327. 

FIGS.  323-327. — Experiment  with  two  Iceland  spar  cleavages. 

the  plane  of  the  long  diagonal  and  for  the  extraordinary  ray  the 
vibrations  are  in  the  plane  of  the  short  diagonal.  See  Figs.  322a 
and  3226. 

If  the  double  image  formed  by  a  piece  of  Iceland  spar  be  viewed 


THE  OPTICAL  PROPERTIES  OF  MINERALS 


171 


through  another  piece  of  Iceland  spar,  in  general  four  images  are 
visible,  pairs  of  which  wax  and  wane  in  turn  as  one  of  the  Iceland 
spars  is  revolved.  In  certain  positions  90°  apart,  only  two 
images  appear.  Figures  323-327  show  diagrammatically  the 
changes  that  take  place.  The  symbols  o  and  e 
refer  to  images  produced  by  'the  first  rhomb, 
while  with  the  second  rhomb  o  gives  rise  to  o0 
and  oe,  and  e  to  e0  and  ee. 

This  behavior  is  good  evidence,  if  not  proof, 
that  in  doubly  refracting  crystals,  light  is  polar- 
ized and  is  vibrating  in  two  planes  which  are  at 
right  angles  to  each  other.  In  Figs.  323-327  the 
short  lines  through  the  circlets  indicate  the  vibra- 
tion planes. 

6.  THE  NICOL  PRISM 

The  principal  device  for  producing  polarized 
light  is  a  Nicol  prism,  so-called  from  the  name  of 
its  inventor,  Nicol.  It  is  a  piece  of  apparatus  to 
which  we  are  indebted  for  much  of  our  knowledge 
of  crystal  optics.  A  clear  piece  of  calcite  or  Ice- 
land spar  (this  variety  is  obtained  almost  exclu- 
sively from  cavities  in  the  basalt  at  a  certain 
locality  in  Iceland)  of  suitable  dimensions  is  cut 
through  in  a  plane  at  right  angles  to  the  principal 
section  and  93°  to  the  terminal  faces.  After 
polishing,  the  two  halves  are  cemented  by  Canada 
balsam  and  mounted.  Figure  328  represents  a 
vertical  cross-section  of  a  Nicol  prism  together 
with  a  horizontal  plan.  Now  as  the  refractive 
index  of  the  ordinary  ray  of  calcite  is  1.658  and 
that  of  the  balsam  about  1.54,  it  will  be  seen  from  the  figure 
that  the  ordinary  ray  o,  in  passing  from  the  calcite  to  the  balsam 
cement  does  not  enter  the  balsam,  but  is  totally  reflected  and 
meets  the  surface  at  an  angle  greater  than  the  critical  angle 


FIG.    328.— Nicol 
prism. 


172         INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


which  is  about  68°.  The  extraordinary  ray,  e,  passes  on  through 
the  balsam  cement  but  slightly  affected  by  the  balsam,  as  for 

this  particular  direction  of  trans- 
mission its  index  of  refraction  is 
1.516.  Hence  there  emerges 
from  the  upper  surface  of  the 
nicol,  plane  polarized  light  with 
vibrations  parallel  to  the  short 
diagonal  of  the  calcite  rhomb  as 
indicated  in  Fig.  328. 

If  a  Nicol  prism  is  examined 
with  a  tourmaline,  darkness  re- 
sults when  the  c-axis  of  the 
tourmaline  is  parallel  to  the  long 
diagonal  of  the  calcite  rhomb. 

If  two  nicols  have  their  short 
diagonals  parallel,  light  goes 
through  unaffected,  except  that 
the  intensity  is  diminished.  If 
one  of  the  nicols  is  revolved,  the 
light  gradually  fades  until  their 
short  diagonals  are  at  right 
angles,  when  darkness  results. 

6.    THE    POLARIZING 
MICROSCOPE 

The  polarizing  microscope, 
also  often  called  the  petrographic 
microscope  (Fig.  329),  differs 
from  an  ordinary  microscope  in 
the  addition  of  a  rotating  stage 
for  measuring  angles  and  of  two 

Nicol  prisms,  one  above,  the  other  below,  the  stage.  Two  nicols 
are  necessary,  for  the  effects  due  to  polarized  light  cannot  usually 
be  distinguished  except  by  another  nicol.  The  lower  nicol  is  called 


Fio.  329. — Polarizing  microscope 
size). 


THE  OPTICAL  PROPERTIES  OF  MINERALS 


173 


the  polarizer  and  the  upper  one,  the  analyzer.  The  lower  one 
fits  into  a  socket  so  it  may  be  rotated,  but  ordinarily  it  is  set  so 
that  its  vibration  plane  is  at  right  angles  to  that  of  the  upper 
nicol.  Then  the  field  should  be  dark  and  the  nicols  are  said  to 
"  crossed." 

In  some  cases  we  use  the  lower  nicol  alone  and  then  its  vibra- 
tion plane  should  be  known.  A  convenient  method  of  making  this 
determination  is  to  examine  crushed  fragments  of  fibrous  tourma- 
line under  the  microscope.  The  tourmaline  prisms  become 
dark  when  their  long  axis  is  perpendicular  to  the  vibration  plane 
of  the  lower  nicol,  as  shown  in 
Fig.  330.  A  thin  section  of 
biotite  may  also  be  used  to  de- 
termine the  vibration  plane  of 
the  lower  nicol.  (See  p.  202.) 

Adjustments  of  the  Polarizing 
Microscope 

The  following  adjustments  of 
the  polarizing  microscope  must 
be  made  in  the  order  indicated. 

1.  " Crossing"  of  the  nicols. 

2.  Determination  of  the  plane 
of  vibration  of  the  lower  nicol. 

3.  Placing  of  the  cross-hairs  of  the  eye-piece  parallel  to  the 
vibration  planes  of  the  nicols. 

4.  Centering  the  stage  (see  below). 

In  order  to  use  the  rotating  stage,  its  center  must  coincide 
with  the  optical  center  of  the  microscope  tube.  The  method  of 
centering  the  stage  may  be  explained  by  referring  to  Fig. 
331.  A  and  B  are  centering  screws  90°  apart,  located  either  on  the 
stage  or  on  the  microscope  tube,  preferably  on  the  latter.  The 
two  perpendicular  lines  across  the  field  represent  the  cross-hairs. 
An  object  o  on  a  glass  slide  is  placed  at  the  intersection  of  the 
cross-hairs,  Suppose  on  revolution  of  the  stage  it  appears  to 


FIG.  330. — Tourmaline   fragments 
polarized  light. 


174        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

revolve  in  the  dotted  circle  until  it  resumes  its  original  position. 
Then  revolve  the  stage  180°,  correct  for  one-half  the  error  by  the 
centering  screws-,  the  other  half  by  moving  the  slide  on  the  stage. 
In  the  figure  the  error  is  oo'>  the  components  of  which  in  the  direc- 
tion of  the  cross-hairs  are  o'e  and  o'd.  They  by  the  centering 
screw  A,  o'  is  first  moved  the  distance  %o'e  and  by  the  centering 
screw  B,  o'  is  moved  the  distance  %o'd.  The  object,  o,  then  takes 
the  position  c  and  the  slide  itself  must  be  moved  the  distance  co, 
when  the  stage  should  be  approximately  centered.  It  may  be 
necessary  to  repeat  the  operation  several  times  for  accurate 
centering. 


FIG.  331. 
Method  of  centering  the  stage. 


FIG,  332. 
Method  of  measuring  an  angle 


To  measure  a  plane  angle  in  a  thin  section  or  the  interfacial 
angle  of  a  small,  flat  crystal  the  stage  is  centered  with  the  inter- 
section of  the  two  edges  at  the  center  of  the  cross-hairs.  A 
reading  is  made  when  one  edge  of  the  crystal  is  parallel  to  the 
east-west  cross-hair,  say,  then  the  stage  is  revolved  until  the 
other  edge  (dotted  line  in  Fig.  332)  is  parallel  to  the  same  cross- 
hair, but  on  the  opposite  side  of  the  center,  when  another  reading 
is  taken.  The  difference  between  the  two  readings  is  the  supple- 
ment or  external  angle  (a  in  the  figure) . 

The  microscope  is  often  used  for  measuring  very  small  distances 


THE  OPTICAL  PROPERTIES  OF  MINERALS  175 

such  as  the  dimensions  of  minute  crystals.  For  this  purpose  a 
special  eye-piece  (micrometer  eye-piece)  containing  a  scale 
etched  on  glass  is  used.  On  the  stage  of  the  microscope  a  scale 
reading  hundred ths  of  a  millimeter  is  placed.  It  is  then 
necessary  to  see  how  many  hundredths  of  a  millimeter  each  divi- 
sion of  the  eye-piece  is  equivalent  to. 

7.  INTERFERENCE  COLORS 

If  thin  plates  of  singly  refracting  (isometric)  crystals  are 
examined  between  crossed  nicols  there  is  no  result,  for  the  original 
field  remains  dark.  But  if  thin  plates  of  doubly  refracting 
crystals  are  examined  between  crossed  nicols  there  result  the 
beautiful  color  effects  known  as  interference  colors.  In  order 
to  explain  these  colors  it  is  necessary  to  consider  the  results 
obtained  in  examining  the  doubly  refracting  plates  in  monochro- 
matic light.  If  two  light  waves  or  train  of  waves  of  the  same 
wave-length  travel  along  the  same  path,  after  they  have  traveled 
different  distances  in  another  medium,  they  combine  in  general 
to  produce  a  new  wave,  the  ordinate  at  any  point  of  which  is  equal 
to  the  sum  of  the  ordinates  of  the  two  original  waves.  This 
phenomenon  is  called  interference.1 

There  are  two  special  cases  of  importance.  (1)  The  two  waves 
have  the  same  amplitude  and  a  path  difference  of  J^X.  As  can  be 
seen  from  Fig.  333  they  neutralize  each  other  and  darkness  is  the 
result.  That  is,  under  certain  conditions  two  light  waves  can 
combine  so  as  to  produce  darkness.  (2)  The  two  waves  have 
the  same  amplitude  and  a  path  difference  of  X.  In  this  case  the 
new  wave  will  have  the  same  wave  length,  but  the  amplitude  will 
be  doubled  (Fig.  334).  For  intermediate  cases  such  as  path  dif- 
ference of  %X,  the  resultant  wave  will  have  an  intermediate 
amplitude  (Fig.  335).  Interference  results  when  light  waves 
from  the  same  source  go  over  the  same  path,  one  in  advance  of  the 
other.  There  are  two  methods  of  obtaining  interference,  one  by 

1  The  interference  in  this  case  is  destructive;  constructive  interference  is 
produced  by  diffraction. 


176        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

the  use  of  thin  films,  the  other  by  the  use  of  doubly-refracting 
crystals. 

Let  Fig.  336  represent  a  thin  film  of  air  in  a  selenite  cleavage. 
AB  and  A'B'  are  oblique  incident  rays.  On  reaching  the  surface 
of  the  film  they  are  partly  reflected  and  partly  refracted.  So 
that  for  a  point  B'  there  will  be  two  rays  traveling  along  B'D'; 
one  of  them  is  the  reflected  ray  of  A'B',  the  other  the  reflected  ray 


Path  difference  =   ^ 


Path  difference  =  — 
FIGS.  333-335. 


of  the  refracted  ray  EC.  These  two  rays  are  in  a  position  to 
interfere  for  one  of  them  has  traveled  a  greater  distance  than  the 
other.  For  while  one  has  traveled  A'B'  the  other  has  traveled 
ABE.  Hence  the  ray  from  AB  is  the  distance  ECB'  behind  the 
ray  from  A'B'. 

If  we  use  monochromatic  light  and  adjust  the  thickness  of  the 
film  so  that  the  retardation,  or  lagging  of  one  ray  behind  the 


THE  OPTICAL  PROPERTIES  OF  MINERALS 


177 


other,  is  equal  to  J^X,  we  have,  for  that  particular  thickness,  dark- 
ness, while  for  a  retardation  of  X  we  have  light  of  maximum  inten- 
sity. Therefore  with  a  wedge-shaped  film  in  monochromatic  light 
we  have  a  series  of  dark  bands,  for  retardations  of  n/2  X  and  n\ 
produce  the  same  effect  as  ^\  and  X  respectively. 

Now  let  us  consider  the  case  'of  doubly-refracting  crystals.  A 
series  of  parallel  light  waves  from  the  polarizer  or  lower  nicol 
enter  the  crystal  and  are  broken  up  into  two  sets  of  waves,  one 
vibrating  in  the  plane  of  the  paper,  say,  and  the  other  in  the 
plane  normal  to  the  paper.  See  Fig.  337.  At  certain  points 
on  the  upper  surface  there  will  emerge  two  sets  of  waves  traveling 


FIG.  336. 


FIG.  337. 


in  the  same  path  but  vibrating  in  planes  at  right  angles  to  each 
other,  and  oblique  to  the  planes  of  vibration  of  the  nicols.  In 
order  that  they  may  interfere  it  is  necessary  to  reduce  the  vibra- 
tions to  one  plane  and  for  this  purpose  an  analyzer  or  upper  nicol 
is  necessary.  The  effects  produced  depend  upon  the  relative 
positions  of  the  nicols,  upon  the  position  of  the  crystal  with 
reference  to  the  nicols,  and  upon  the  path  difference  of  the  two 
sets  of  polarized  light  waves. 

Figure  338  explains  diagrammatically  what  happens  when  a 
doubly  refracting  crystal  is  examined  between  the  crossed  nicols 
of  a  polarizing  microscope.  A  ray  of  light  entering  the  lower 


12 


178        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

nicol  is  broken  into  two  rays,  e  and  o.  One  of  these  (o)  is  totally 
reflected  and  disappears.  The  remaining  ray  (e)  is  broken  into 
two  rays  (e'  and  o')  by  the  mineral  plate.  These  two  rays,  which 


FIG.  338. — Diagrammatic  representation  of  doubly  refracting  crystal  examined 
between  crossed  nicols. 

are  vibrating  at  right  angles  to  each  other,  are  each  broken  into 
two  rays  (e",  o",  and  e'" ,  o'"),  by  the  upper  nicol.  One  from 
each  of  these  (o"  and  o'"}  is  totally  reflected  and  finally  there 
emerge  from  the  top  of  the  upper  nicol  two  sets  of  waves  (e"  and 


THE  OPTICAL  PROPERTIES  OF  MINERALS 


179 


e'")  vibrating  in  the  same  plane  and  these  interfere  with  each 
other. 

With  crossed  nicols  darkness  results  when  the  path  difference 
is  X.  In  Fig.  339,  PP'  is  the  vibration  plane  of  the  lower  nicol  and 
AA'  of  the  upper  nicol,  RR'  and  SS',  the  vibration  planes  of  the 
crystal  plates.  The  distances  or  and  os  represent  vibrations  in 
the  same  phase.  The  components  of  these  in  the  plane  of  the 
upper  nicol  are  op  and  ocr,  which  are  opposite  and  equal.  Hence 


FIG.  339. 


FIG.  340. 


FIG.  341. 


they  annul  each  other.  With  crossed  nicols  there  is  maximum 
light  when  the  path  difference  is  J^X.  In  Fig.  341,  r'  and  s 
represent  vibrations  of  J^X  path  difference.  Their  components 
in  the  plane  of  the  upper  nicol  are  p'  and  a,  equal  but  on  the 
same  side  of  the  origin.  Hence  the  intensity  is  doubled.  For  a 
path  difference  of  J^X,  we  have  the  intensity  shown  in  Fig.  340. 
Thus  the  intensity  varies  between  0  for  a  retardation  of  X,  and  a 
maximum  for  retardation  of  J^X. 

As  can  be  seen  from  Fig.  342,  the  retardation  produced  by  a, 


180        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

wedge-shaped  section  of  a  crystal  will  vary  from  point  to  point. 
A  retardation  of  n\  will  give  darkness  and  a  retardation  of  n/2  X  will 
give  a  maximum  intensity.  Hence  a  wedge  examined  in  mono- 
chromatic light  beween  crossed  nicols  will  appear  as  a  series  of 
parallel  dark  bands  interspersed  with  colored  spaces. 

For  white  light  we  have  the  combined  effect  of  all  the  colors 
of  the  spectrum.  Some  idea  of  the  interference  colors  seen  in 
white  light  may  be  gained  by  a  study  of  Fig.  343.  (This  diagram 
may  be  colored  by  the  student.)  The  top  of  the  figure  represents 
a  wedge-shaped  section  as  viewed  in  various  kinds  of  monochro- 
matic light,  there  being  a  dark  band  at  positions  which  give 
retardations  of  n\.  For  each  of  these  colors  a  medium  value  of 


FIG.  342. — Wedge  of  a  doubly-refracting  crystal  between  crossed  nicols. 

the  wave  length  is  chosen  as  follows:  red,  700^;  orange,  620/x/x; 
yellow,  560juju;  green,  515MMJ  blue,  460/xju;  violet,  410/^/4.  The 
top  row  of  figures  gives  the  value  of  retardations  in  n,u. 

The  lower  part  of  the  figure  indicates  the  interference  colors  as 
seen  in  white  light.  Let  us  consider  the  colors  in  succession. 
The  intensity  of  different  parts  of  the  spectrum  varies  and  has  an 
influence  in  determining  the  color.  Yellow-green  is  the  most 
intense,  and  violet,  the  least  intense  part  of  the  spectrum.  The 
interference  color  chart,  as  it  is  called,  begins  with  darkness, 
succeeded  by  dark  gray  which  gradually  becomes  lighter.  At 
about  250jUM  all  the  colors  combine  to  form  white  light.  At 
for  yellow)  yellow  is  at  a  maximum,  but  mixed  with 


THE  OPTICAL  PROPERTIES  OF  MINERALS 


181 


A    100  200  300  400  500  800  700  800  900  1000  1100  1200  1300  1400  1500  1000  1700  1300  1900  2000  2100 


Orange 


Je'low 


Green 


Blue 


Violet 


*  ? 

A    o 


is!  s  I|  1 


2     2 


100    200     300   400     500    600    700    800    900   1000  1100  1200  1300  1400 1500  1800  1700  1300 1900  2000  2100 
First  Order  Second  Order  Third  Order  Fourth  Order 


FIG.  343. — The  derivation  of  the  interference  color  chart. 

white  it  gives  straw  yellow.  At  310/i/i  and  at  350/i/i,  orange  and 
red  respectively,  are  at  a  maximum,  but  the  great  intensity  of  the 
yellow  modifies  these  colors  and  places  them  further  to  the  right, 
for  at  360/iM  the  color  is  bright  yellow.  At  about  550juM>  violet 
is  the  color.  Though  of  weak  intensity,  violet  is  produced  here 
because  the  other  colors  are  practically  extinguished.  As  can  be 
seen  from  the  diagram  the  colors  follow  in  order:  blue,  green, 
yellow,  and  red.  At  about  11 00/-iM>  violet  appears  again.  At  this 
point  only  red,  blue,  and  violet  are  near  a  maximum.  But  red 
and  blue  together  produce  violet.  Then  in  order  we  have  blue, 
green,  yellow,  and  red  again.  These  same  colors  are  repeated 
a  second  time  but  become  paler  and  then  pass  into  neutral 
tints  (largely  pink  and  pale  green)  and  finally  into  high-order 
white  which  resembles  ordinary  white  light. 


182        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

There  is  a  repetition  in  the  colors,  but  they  gradually  become 
fainter.  The  colors  from  black  up  to  the  first  violet  (A  =  550/i/i) 
are  called  first -order  colors,  from  this  violet  up  to  the  second 
violet  (A=  1100/i/z),  second -order  colors,  and  so  on.  In  white 
light  seven  err  eight  orders  may  be  distinguished,  but  in  mono- 
chromatic light  there  is  no  limit  to  the  number  of  orders  as 
determined  by  the  dark  bands. 

By  trial  it  may  be  found  that  the  interference  color  depends 
upon  three  factors:  (1)  the  double  refraction  which  is  a  constant 
for  the  crystal,  (2)  the  orientation  or  direction  in  which  the 
crystal  is  cut  (e.g.,  in  a  thin  section  of  sandstone  the  quartz  grains, 
cut  in  various  directions,  have  a  great  variety  of  interference 
colors),  (3)  the  thickness,  as  may  be  seen  in  a  quartz  or  gypsum 
wedge. 

The  formula  A  =  t(ni  —  n^)  gives  the  relation  between  A,  the 
retardation  in  W,  t,  the  thickness  of  the  plate,  and  (n\  —  n2),  the 
double  refraction ;  HI  and  n2  are  the  two  values  of  the  indices  of 
refraction  for  a  particular  section.  For  a  given  substance  with 
known  indices  of  refraction  the  thickness  may  be  measured  and 
the  interference  colors  predicted.  Or  the  thickness  may  be 
measured,  the  retardation  determined  from  the  color  chart,  and 
the  double  refraction  calculated.  Or  the  thickness  may  be  cal- 
culated that  will  give  a  certain  interference  color  for  a  crystal 
with  known  double  refraction.  In  the  interference  color  chart 
of  Fig.  344,  the  horizontal  lines  represent  the  thickness  from 
0.00  to  0.06  mm.;  ordinary  rock  slides  and  fragments  are 
from  0.03  to  0.05  mm.  in  thickness.  The  vertical  lines  give 
retardations  in  /*/*,  while  the  diagonal  lines  represent  the  amount 
of  the  double  refraction.  A  crystal  of  0.03  mm.  thickness  and 
double  refraction  of  0.02  has  a  retardation  of  600^  (0.000600  = 
0.03  X  0.02),  and  gives  a  second-order  indigo  blue  interference 
color.  Fig.  344  may  be  colored  to  correspond  to  the  colors  given 
at  the  bottom  of  Fig.  343;  the  names  of  the  common  minerals 
may  be  written  in  the  blank  space  to  the  left  in  the  appropriate 
position  according  to  the  double  refraction. 


183 


184        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

8.  VIBRATION  OR  EXTINCTION  DIRECTIONS 

If  a  section  of  a  doubly  refracting  crystal  is  revolved  between 
crossed  nicols  there  is  darkness  four  times  in  a  complete  revolu- 
tion. This  is  called  extinction  and  is  simply  due  to  the  fact  that 
for  these  particular  positions  the  crystal  has  no  effect  upon  the 
dark  field  produced  by  the  crossed  nicols.  The  two  directions  in 
the  crystal  parallel  to  the  vibration  planes  of  the  two  nicols  are 
called  extinction  directions  or  vibration  directions.  These  two 
directions  are  directions  of  the  two  plane  polarized  waves  pro- 
duced in  doubly  refracting  crystals  by  the  plane-polarized  light 
of  the  lower  nicol. 


FIG.  345. 
Parallel  extinction. 


FIG.  346. 
Oblique  extinction. 


FIG.  347. 
Symmetrical  extinction. 


Now  according  to  the  position  of  these  directions  with  refer- 
ence to  the  crystal  outlines  we  have  various  kinds  of  extinction 
characteristic  of  crystals  of  the  various  systems.  In  case  the 
directions  are  parallel  to  the  outline  we  have  parallel  extinction. 
This  is  represented  by  the  convention  of  Fig.  345,  the  cross-hairs 
of  the  microscope  being  parallel  to  the  vibration-planes  of  the 
nicols,  and  the  crystal  placed  in  the  position  of  darkness.  If 
the  directions  are  not  parallel  to  the  outline  we  have  oblique 
extinction  (Fig.  346).  The  particular  case  in  which  these  direc- 
tions make  equal  angles  with  the  edges  of  the  crystal  is  called 
symmetrical  extinction  (Fig.  347). 


THE  OPTICAL  PROPERTIES  OF  MINERALS  185 

The  angle  between  an  extinction  direction  and  a  prominent 
crystallographic  direction  of  a  crystal  (usually  the  c-axis)  is 
called  the  extinction  angle,  and  is  characteristic  of  certain  crystals 
in  certain  directions.  The  extinction  angle  is  determined  by 
taking  a  reading  when  the  outline  is  parallel  to  one  of  the  cross- 
hairs (the  stage  being  centered)  and  then  revolving  the  stage 
until  maximum  darkness  results,  when  another  reading  is  taken. 
The  difference  between  the  two  readings  is  the  extinction  angle. 
In  Fig.  346,  the  extinction  angle  indicated  by  the  arrow  is  15°. 
It  may  be  noticed  that  there  are  two  possible  extinction  angles 
which  are  complementary.  The  smaller  angle  ( <  45°)  is  usually 
taken. 

Accurate  determinations  of  the  extinction  angle  are  made  in 
monochromatic  light.  A  convenient  determination  in  white 
light  may  be  made  by  using  a  gypsum  plate  which  gives  a  field 
showing  red  of  the  first  order.  This  is  called  the  sensitive  tint, 
for  the  least  change  gives  either  orange-red  or  violet-blue.  When 
inserted  in  the  slot  provided  for  test-plates,  a  doubly  refracting 
crystal  appears  the  same  tint  of  red  as  the  red  field  only  when 
it  is  in  the  extinction  position. 

9.    THE   DETERMINATION   OF  THE  INDICES  OF  REFRACTION  IN 
DOUBLY-REFRACTING  CRYSTALS 

For  a  section  of  a  doubly-refracting  crystal  cut  in  any  direction 
there  are  in  general  two  values  of  the  index  of  refraction,  one  for 
each  of  the  two  vibration  directions  at  right  angles  to  each  other. 
These  two  directions  are  the  extinction  positions  for  the  particular 
section.  After  bringing  the  crystal  plate  or  fragment  into 
extinction,  one  index  of  refraction  is  determined  with  reference 
to  the  set  of  liquids  described  on  p.  166.  Then  after  revolving 
the  stage  of  the  microscope  90°,  the  other  index  is  determined 
in  exactly  the  same  way.  See  Fig.  348. 

The  two  values  of  the  index  of  refraction  may  be  designated 
fti  and  r&2,  and  their  difference  (n\  —  n2)  is  the  double  refraction 
or  birefringence  for  the  particular  section.  The  vibration  direc- 


186        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


tion  of  the  two  rays  of  the  doubly-refracting  substance  are  at 
right  angles  to  each  other. 

For  a  given  doubly-refracting  crystal  cut  in  various  direction 
a  good  many  different  values  of  HI  and  n2  may  be  obtained.  The 
maximum  of  all  possible  values  of  n\  is  denoted  by  ny,  and  the 
minimum  of  all  possible  values  of  n2  is  denoted  by  na.  Some 
particular  section,  which  may  be  recognized  by  the  fact  that  it 
has  the  highest  interference  color  for  a  given  thickness,  will 

furnish  both  ny  and  na.  In 
addition  to  ny  and  na,  a  great 
many  intermediate  values  may 
be  obtained.  In  orthorhombic, 
mono  clinic,  and  triclinic 
crystals  (these  are  collectively 
called  biaxial)  a  section  cut 
normal  to  the  plane  of  a  and  y 
furnishes  an  important  inter- 
mediate (not  a  mean)  value 
known  as  %.  The  fact  that 
the  value  np  always  lies  between 
FIG.  348. — To  illustrate  the  deter-  (except  in  the  rare  cases  in 

mination  of  indices  of  refraction  in  wnich  it  is  exactly  equal  to  one 
doubly-refracting  crystals.  , 

or  both  of  them)  the  two  values 

of  HI  and  nz  found  on  any  section  enables  one  to  determine  it 
by  trying  fragments  in  various  liquids  until  a  liquid  is  found,  the 
index  of  refraction  (n3)  of  which  is  greater  than  that  for  various 
fragments  (nz>ni>nz]  and  also  another  liquid,  the  index  of 
refraction  (n*)  of  which  is  less  than  that  of  various  fragments 
(n4<ri2<fti).  Then  np  lies  between  n3  and  n\. 

The  above  discussion  implies  that  the  mineral  fragments  have 
no  cleavage,  and  hence  all  possible  orientations  are  obtained. 
If,  however,  a  mineral  has  good  cleavage,  it  may  not  be  possible  to 
determine  the  three  principal  indices  of  refraction  na,  n^  and  ny. 
In  colemanite,  for  example,  the  two  values  np  and  ny  are  obtained, 
but  not  na,  on  account  of  the  good  cleavage  parallel  to  (010). 


THE  OPTICAL  PROPERTIES  OF  MINERALS  187 

10.  DIRECTION  OF  THE  FASTER  AND  SLOWER  RAY 

For  a  section  of  a  doubly-refracting  crystal  cut  in  any  direction 
there  are  in  general  two  values  of  the  index  of  refraction  corre- 
sponding to  the  two  vibration  directions  at  right  angles  to  each 
other.  One  of  the  values  is  greater  than  the  other,  otherwise 
there  would  be  no  double  refraction.  Interference  in  doubly- 
refracting  crystals  is  caused  by  one  ray  getting  behind  the  other. 
The  one  that  is  retarded  is  called  the  slower  ray,  the  other,  the 
faster  ray.  The  ray  with  the  greater  index  of  refraction  is  the 
slower  ray,  and  the  one  with  the  smaller  index  of  refraction  is 
the  faster  ray. 


FIG.  349.— Faster  ray.  FIG.  350. — Slower  ray. 

FIGS.  349-350. — To  show  the  reciprocal  relation  of  velocity  and  index  of 

refraction. 

This  reciprocal  relation  of  velocity  and  index  of  refraction 
may  be  proved  by  means  of  Figs.  349  and  350.  The  two  figures 
are  sections  taken  at  right  angles  to  each  other.  Figure  349  re- 
presents the  faster  ray  and  Fig.  350  the  slower  ray.  In  Fig.  349 
the  index  of  refraction  is  about  1.33  and  the  velocity  is  V\  in 
terms  of  V,  the  velocity  in  air,  while  in  Fig.  350  the  index  of  re- 
fraction is  about  1.92  and  the  velocity  is  Vn  in  terms  of  V. 

The  determination  of  the  faster  and  slower  ray  may  be  made 
by  determining  the  indices  of  refraction  as  outlined  in  the  pre- 
ceding section,  but  this  is  not  always  convenient.  Another 
method  is  based  upon  the  fact  that  the  superposition  of  one 


188        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


doubly-refracting  section  on  another  has  the  same  effect  as  thicken- 
ing or  thinning  the  section  by  causing  the  interference  colors  to 
go  up  or  down  on  the  interference  color  chart;  "up"  is  toward 
the  thick  end  of  a  wedge  and  "down"  toward  the  thin  end  of  a 
wedge.  Several  test-plates  are  used  for  this  purpose.  The  one 
most  frequently  used  is  the  mica  plate,  a  cleavage  of  muscovite 
of  a  thickness  sufficient  to  produce  a  retardation  of  140/iM 
(J4\  for  medium  yellow,  and  hence  called  the  quarter-undulation 
mica  plate).1  The  direction  of  the  slower  ray  of  this  plate  is 
marked  by  an  arrow.  (This  direction  is  the  line  joining  the 


FIG.  351. — Positive  elongation. 


FIG.  352. — Negative  elongation. 


branches  of  the  hyperbola  of  the  interference  figure  seen  in  con- 
vergent light.)  The  mica  plate  itself  gives  a  pale  bluish-gray 
interference  color  of  the  first  order.  When  placed  in  the  slot 
provided  for  the  purpose  in  the  lower  end  of  the  microscope  tube 
it  causes  the  interference  color  of  a  crystal  section  placed  in  the 
position  of  maximum  illumination  to  be  lowered  or  raised  by 
140/AM  (see  color  chart  p.  181).  The  color  goes  "up"  when 
similar  directions  are  parallel  (when  slower  ray  of  crystal  coincides 
with  slower  ray  of  the  mica  plate)  and  "down"  when  dissimilar 
directions  are  parallel  (when  slower  ray  of  crystal  coincides  with 
faster  ray  of  the  mica  plate  or  vice  versa). 

1  The  correct  thickness  of  this  plate  may  be  judged  by  the  fact  that  the  first  ring  of  the 
interference  figure  is  a  complete  ellipse. 


THE  OPTICAL  PROPERTIES  OF  MINERALS  189 

This  test  is  often  employed  to  determine  the  elongation  of 
the  crystal,  that  is,  to  find  whether  the  long  direction  of  the 
crystal  is  the  slower  ray  or  the  faster  ray.  Examples  of  the  two 
cases  are  given  in  Figs.  351  and  352,  the  dotted  rectangle  repre- 
senting the  mica  plate  with  arrow  indicating  the  slower  ray. 
In  the  first  case,  the  crystal  originally  with  red  interference  color 
is  changed  to  blue,  when  its  length  is  parallel  to  the  arrow  of  the 
mica  plate.  Hence  the  elongation  is  parallel  to  the  slower  ray. 
This  is  called  positive  elongation.  In  the  other  case,  the  red  crys- 
tal is  changed  to  blue,  when  its  length  is  parallel  to  the  faster  ray 
of  the  mica  plate,  (the  faster  ray  is  always  perpendicular  to 
the  slower  ray).  This  is  called  negative  elongation. 

11.  CLASSIFICATION  OF    CRYSTALS  FROM  AN  OPTICAL  STAND- 
POINT 

From  an  optical  standpoint  there  are  three  divisions  of  crystals: 
isotropic  (isometric),  uniaxial  (tetragonal  and  hexagonal),  and 
biaxial  (orthorhombic,  monoclinic,  and  triclinic).  In  discussing 
the  optical  properties  it  is  convenient  to  employ  a  geometrical 
representation  of  the  optical  structure.  The  figure  formed  by 
taking  as  radius  vector  the  index  of  refraction  in  various  direc- 
tions is  called  the  optic  ellipsoid.1  The  optic  ellipsoid  is,  in 
general  a  triaxial  ellipsoid,  sections  of  which  are  ellipses.  The 
important  property  of  the  ellipsoid  is  that  the  major  and  minor 
axes  of  any  elliptic  section  or  optic  ellipse  are  the  extinction  direc- 
tions for  that  section  and,  moreover,  the  lengths  of  these  axes  are 
proportional  to  the  indices  of  refraction  for  that  particular  section. 
The  shape  of  the  ellipsoid  varies  for  the  three  divisions  of  crystals 
mentioned. 

In  isometric  crystals  the  index  of  refraction  is  the  same  for 
all  directions.  The  optic  ellipsoid  is  therefore  a  sphere.  All 
sections  of  a  sphere  are  circular,  so  there  is  no  double  refraction 

1  There  are  a  number  of  other  ellipsoids  used  in  crystal  optics  but  since  the  index  of  re- 
fraction is  the  most  fundamental  optical  constant,  the  above  designated  one  may  be  called 
the  optic  ellipsoid. 


190        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

and,  hence,  no  interference  colors.  A  section  cut  in  any  direction 
will  remain  dark  between  crossed  nicols.  Isometric  crystals 
and  amorphous  substances  such  as  glass  are  said  to  be  isotropic. 
Crystals  of  the  remaining  systems  (those  except  the  isometric) 
have  double  refraction;  for  these  the  term  anisotropic  is  used 
in  contrast  with  the  term  isotropic. 

Tetragonal  and  hexagonal  crystals  constitute  the  uniaxial  divi- 
sion. If  sections  of  these  crystals,  cut  in  various  directions,  are 
examined  between  crossed  nicols,  it  is  found  that  basal  sections 
(sections  perpendicular  to  the  c-axis)  remain  dark  between  crossed 
nicols,  and  that  all  sections  parallel  to  the  c-axis  give  some  inter- 
ference color  which  is  a  maximum  for  a  given  thickness,  while 
sections  oblique  to  the  c-axis  give  interference  colors  varying 
from  a  maximum  to  darkness  (the  color  depends  upon  the  ob- 
liquity) ,  but  those  of  equal  obliquity  to  the  c-axis  give  the  same 
interference  color. 

From  these  tests  it  will  be  seen  that  the  optic  ellipsoid  is  an 
ellipsoid  of  revolution.  The  axes  of  an  elliptical  section  through  the 
c-axis  represent  the  indices  of  refraction,  one  of  which  (designated 
ft7) ,  is  the  maximum  of  all  possible  values  in  the  crystal,  while  the 
other  (designated  na),  is  the  minimum  of  all  possible  values.  In 
some  cases  7,1  or  the  slowest  ray,  is  parallel  to  the  c-axis,  while  in 
other  cases,  a,  or  the  fastest  ray,  is  parallel  to  the  c-axis.  This 
divides  the  uniaxial  crystals  into  two  divisions,  the  optically 
positive  (c  =  7)  and  the  optically  negative  (c  =  a) ;  here  the  terms 
positive  and  negative  are  purely  arbitrary.  The  ellipsoid  of  posi- 
tive crystals  is  in  reality  an  oblate  spheroid  and  that  of  negative 
crystals,  a  prolate  spheroid,  because  the  extreme  values  of  the 
indices  of  refraction  are  not  very  different  except  in  a  few  cases 
such  as  that  of  calcite. 

The  determination  of  the  optical  character,  that  is,  whether 
positive  or  negative,  is  made  by  ascertaining  the  faster  and  slower 
ray  in  a  section  parallel  to  the  c-axis,  or  in  a  basal  section  by 

1  a  and  y  are  directions  in  a  crystal,  and  na  and  ny  are  indices  of  refraction  for  these 
directions. 


THE  OPTICAL  PROPERTIES  OF  MINERALS 


191 


BXa 


testing  the  interference  figure,   obtained  by  convergent  light, 
with  a  mica  plate. 

Crystals  of  the  remaining  systems,  orthorhombic,  monoclinic, 
and  triclinic,  constitute  the  biaxial  division.  Of  all  the  possible 
values  of  the  indices  of  refraction  in  a  biaxial  crystal  it  is  found 
that  one  section  contains  the  direction  7  corresponding  to  the 
maximum  index  of  refraction  nyj  and  also  the  direction  a  corre- 
sponding to  the  minimum  index  of  refraction  na.  For  other  sec- 
tions the  indices  of  refraction  are  in- 
termediate between  the  maximum  and 
minimum.  The  direction  perpendicu- 
lar to  the  plane  of  7  and  a  is  called  /3, 
or  sometimes  the  optic  normal.  The 
index  of  refraction  n$  is  intermediate 
between  ny  and  na,  but  is  not  a  mean 
value  and  is  recorded  simply  because 
it  is  one  of  the  values  for  sections  cut 
normal  to  a  and  to  7. 

The  optic  ellipsoid  for  biaxial 
crystals  made  by  laying  off  on  three 
rectangular  axes  the  values  na,  n^  and 
ny  (indices  of  refraction  for  the  three 
directions  mentioned)  is  a  triaxial 
ellipsoid.  The  maximum  interference 
color  for  a  given  thickness  is  given  by 

-.-.-.  ' 

the  section  which  includes  7  and  a. 
The  interference  colors  vary  from  this  maximum  to  a  minimum, 
but  there  is  no  section  that  remains  dark  between  crossed  nicols 
as  in  uniaxial  crystals. 

Figure  353  is  the  section  of  a  triaxial  ellipsoid  which  contains  7 
and  a.  There  are  two  circular  sections  of  this  ellipsoid.  Lines 
normal  to  these  circular  sections  are  peculiar  directions  corre- 
sponding somewhat  to  the  single  direction  or  c-axis  of  uniaxial 
crystals.  These  directions  are  called  optic  axes,  hence  the 
term  biaxial.  A  plate  of  a  biaxial  crystal  normal  to  an  optic 


FIG.  353.—  Section  of  the  optic 
<&&?<>&  of  a  biaxial  crystal  con- 

taming  a,  7,  and  the  optic  axes. 


192 


INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


axis  appears  uniformly  bright  between  crossed  nicols  for  all 
positions  of  rotation.  The  optic  axes  always  lie  in  the  plane  of  7 
and  a,  which  is  therefore  called  the  plane  of  the  optic  axes  or  the 
axial  plane;  the  acute  angle  between  the  optic  axes  is  called 
the  axial  angle  (denoted  by  the  term  2F).  In  Fig.  353,  OA  and 
OA'  are  the  optic  axes  and  AOA'  the  axial  angle.  The  optic 
axes  are  always  symmetrically  placed  with  respect  to  7  and  a. 

SYNOPSIS  OF  THE  OPTICAL  PROPERTIES  AND  CONSTANTS  FOR  THE  CRYSTAL 

SYSTEMS 


g 

«1  S 

All  sections 

No  inter- 

i 

ft  ft  "p. 

ISOMETRIC 

remain  dark 

n 

ference 

1 

03  « 

figure 

c 

1 

I 

TETRAGONAL 

UNIAXIAL 
soid  a  spheroid  of 

Basal  sec- 
tion dark. 
Parallel  ex- 
tinction in 
most  other 
sections 

na,  ny 

Interference 
figure   with 
dark  cross 
and  colored 
circles 

p. 

3 

HEXAGONAL 

0 

£ 
o 

1 

Parallel  ex- 

Symmetrical 

% 

ORTHORHOM- 

tinction  in 

dispersion. 

BIC 

most  sec- 

tions. 

! 

Parallel  ex- 

Interference 

Horizontal, 

j  '^ 

tinction  in 

figure  with 

inclined,  or 

3  'C 

100,  001, 

dark  hyper- 

crossed 

35 

MONOCLINIC 

and  hOl 

na,  n0,  ny 

bola  and 

dispersion. 

2V 

«  ^ 

sections. 

colored 

I 

Oblique  in 

ellipses  and 

13 

others. 

lemniscates. 

Q 

I 

TRICLINIC 

Oblique  ex- 

Asymmetric 

0 

tinction  in 

dispersion. 

all  sections 

THE  OPTICAL  PROPERTIES  OF  MINERALS  193 

The  line  bisecting  the  axial  angle  is  called  the  acute  bisectrix 
(denoted  by  Bxa).  The  line  bisecting  the  obtuse  angle  between 
the  optic  axes  is  called  the  obtuse  bisectrix  (denoted  by  Bx0). 

There  are  two  divisions  of  biaxial  crystals  according  to  whether 
the  acute  bisectrix  is  7  or  a.  The  former  are  called  positive 
(Bxa  =  7),  and  the  latter,  negative  (Bxa  =  a);  this  is  a  purely 
arbitrary  designation.  Figure  353  represents  a  positive  crystal. 

The  determination  of  the  optical  character  may  be  made  by 
testing  for  the  faster  and  slower  ray  in  a  section  known  to  be 
perpendicular  to  the  acute  bisectrix.  It  may  also  be  determined 
in  this  kind  of  section  by  obtaining  an  interference  figure  and 
testing  it  with  a  mica  plate. 

The  optical  properties  for  the  crystal  systems  may  be  tabu- 
lated as  in  the  preceding  table. 

12.  INTERFERENCE  FIGURES 

The  tests  mentioned  up  to  this  point  have  been  made  by 
using  ordinary  parallel  light  or  polarized  parallel  light.  A  unique 
series  of  effects,  important  in  the  identification  and  description 
of  minerals,  may  be  obtained  by  examining  suitable  sections  in 
convergent  polarized  light.  For  this  purpose  either  a  polari- 
scope  or  a  polarizing  microscope  may  be  used.  A  polariscope 
is  an  instrument  consisting  essentially  of  an  analyzer  and  a  polarizer 
with  slight  magnifying  power  and  strongly  convergent  lenses  both 
above  and  below  the  stage. 

If  the  polarizing  microscope  is  used,  a  high  power  objective 
(Focal  length  =  3  to  5  mm.)  and  also  a  condensing  lens  placed 
just  below  the  stage  must  be  substituted  for  the  ordinary  set-up. 
Either  the  eye-piece  must  be  removed,  or  a  special  lens,  called 
the  Bertrand  lens,  must  be  inserted  in  the  microscope  tube  be- 
tween the  analyzer  and  the  eye-piece. 

The  color  effects  seen  when  basal  sections  of  uniaxial  crystals 
and  sections  of  biaxial  crystals  cut  normal  to  the  acute  bisectrix, 
are  examined  in  convergent  light  between  crossed  nicols  are 
known  as  interference  figures. 

13 


194        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


FIG.  354. 


FIG.  355. 


FIG.  356.  FIG.  357. 

FIGS.  354-357. — INTERFERENCE  FIGURES  OBTAINED  BETWEEN  CROSSED  NICOLS  IN  MONC 

CHROMATIC  LIGHT.     (After  Hauswaldt.} 

FIG.  354. — Uniaxial  interference  figure  (calcite).     Plate  cut  normal  to  the  optic  axis, 
FIG.  355. — Uniaxial  interference  figure  (calcite).     Plate  cut  oblique  to  the  optic  axis. 
FIG.  356. — Biaxial   interference    figure    (aragonite).     Plate    cut   normal   to   the   acut 
bisectrix.     Normal  position. 

FIG.  357. — Biaxial    interference    figure    (aragonite).     Plate   cut    normal  to   the  acut 
bisectrix.     Diagonal  position. 


THE  OPTICAL  PROPERTIES  OF  MINERALS 


195 


With  isometric  crystals  no  interference  figures  are  obtained, 
for  there  is  no  double  refraction.  Double  refraction  is  necessary 
for  the  production  of  interference  figures.  In  fact,  interference 
figures  are  simply  the  result  of  interference  colors,  due  to  varying 
double  refraction  in  different  directions,  combined  and  modified 
by  the  darkness  due  to  crossed'nicols. 


FIG.  358. — Explanation  of  a  uniaxial  interference  figure. 

Basal  sections  of  uniaxial  crystals  examined  in  monochromatic 
convergent  light  between  crossed  nicols  give  a  dark  cross  with 
dark  concentric  rings  (Fig.  354).  The  explanation  is  as  follows: 
(see  Fig.  358).  Strongly  convergent  rays  of  light  traverse  the 
crystal  in  various  oblique  directions  and  the  effect  is  the  same 
as  if  rays  of  parallel  light  were  transmitted  through  a  wedge 
of  the  crystal.  Therefore,  along  any  radius  we  get  a  dark  band 


196        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

where  the  retardation  is  n\  for  the  particular  light  used.  Mid- 
way between  the  bands  we  get  the  maximum  color.  As  the 
structure  is  the  same  all  around  the  c-axis  in  uniaxial  crystals,  the 
dark  bands  are  circular.  It  remains  to  explain  the  dark  cross, 
which,  it  should  be  noticed,  is  stationary  on  rotation  of  the  section 
on  the  stage.  The  crystal,  it  may  be  imagined,  is  made  up  of 
innumerable  parts,  each  with  extinction  directions  at  right  angles. 
These  parts  are  arranged  radially  around  a  center,  and  on  rota- 
tion of  the  stage,  as  vibration  directions  of  successive  parts  be- 
come parallel  to  the  vibration  directions  of  the  nicols,  darkness 
results  for  that  particular  part.  As  new  radii  are  always  coming 
into  the  extinction  position  there  is  always  a  dark  cross,  the  arms 
of  which  are  parallel  to  the  vibration  directions  of  the  two  nicols 
and  so  remain  fixed.  That  the  optic  axis  is  simply  a  direction 
in  the  crystal  may  be  proved  by  the  fact  that  the  interference 
figure  remains  the  same  when  the  crystal  plate  is  moved  about  on 
the  microscope  stage. 

For  sections  not  quite  parallel  to  the  basal  plane,  the  dark  cross 
on  rotation  of  the  stage  is  eccentric,  but  the  arms  always  remain 
parallel  to  the  vibration  planes  of  the  nicols  and  revolve  in  the 
same  direction  in  which  the  stage  is  rotated  (see  Fig.  355). 

With  ordinary  white  light  we  still  have  the  black  cross,  but  the 
dark  rings  become  colored  rings,1  the  colors  of  which  vary  from 
black  at  the  center,  through  gray,  white,  yellow,  red,  blue,  green 
and  so  on,  until  after  six  or  seven  orders  there  is  practically  white 
light.  The  number  of  rings  depends  upon  the  thickness  and  also 
upon  the  double  refraction.  In  very  thin  sectons  there  may  be 
no  rings  visible.  Very  thick  sections  show  the  full  number  of 
rings.  Quartz  with  weak  double  refraction  shows  for  ordinary 
thickness  no  rings  at  all,  while  calcite  with  very  strong  double 
refraction  shows  a  large  number  of  rings. 

The  optical  character  of  a  uniaxial  crystal  may  be  determined 
from  the  interference  figure  by  inserting  in  the  slot  of  the  micro- 

:  The  rings  may  appear  to  be  dark  on  account  of  the  weak  intensity  of  the  violet  portions 
but  the  borders  at  least  are  colored. 


THE  OPTICAL  PROPERTIES  OF  MINERALS  197 

scope  tube  just  above  the  objective,  a  mica  plate  with  the  slower 
ray,  7,  in  the  45°  position.  The  interference  figure  is  changed, 
the  dark  cross  disappears  and  two  dots  appear  in  two  oppo- 
site quadrants,  as  represented  diagrammatically  in  Fig.  359. 
If  the  imaginary  line  joining  the  two  dots  is  perpendicular  to 
7  of  the  mica  plate,  the  crystal  is  positive  (as  in  Fig.  359a),  while 
if  parallel  to  7  of  the  mica  plate,  it  is  negative  (as  in  Fig.  3596). 
This  is  due  to  the  fact  that  the  interference  colors  "  go  up  "in  two 
opposite  quadrants  and  "go  down"  in  the  other  two  quadrants. 
The  rings,  then,  are  not  continuous,  but  broken,  and  the  two 
rings  nearest  the  center  form  the  two  dots. 


FIG.  359a.  FIG.  3596. 

FIGS.  359a-3596. — Uniaxial  interference  figures  with  superimposed  mica  plate. 

Sections  of  biaxial  crystals  cut  normal  to  the  acute  bisectrix 
show  an  interference  figure  like  that  of  Fig.  356,  with  a  black 
cross  and  two  sets  of  concentric  ellipses  passing  into  8-shaped 
curves  (lemniscates) .  In  monochromatic  light  the  rings  are 
dark  and  in  white  light,  colored.  On  revolving  the  section  on  the 
stage,  the  dark  cross  opens  up  and  passes  into  hyperbolae  as 
shown  in  Fig.  357,  which  represents  the  45°  position.  The  line 
joining  the  centers  of  the  ellipses  is  the  trace  of  the  axial  plane; 
the  centers  of  the  ellipses  represent  the  emergence  of  the  optic 
axes. 

The  biaxial  interference  figure  may  be  explained  by  means  of 
the  diagrammatic  Fig.  360.  For  monochromatic  light  the  optical 
structure  of  biaxial  crystals  is  symmetrical  to  three  planes  at 


198        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

right  angles  to  each  other.  One  of  these  is  the  plane  of  the  paper, 
while  the  other  two  are  represented  by  their  traces,  the  vertical 
and  horizontal  lines  of  the  figure,  which  are  also  vibration  planes 
of  the  two  nicols.  The  two  circlets  are  traces  of  the  optic  axes. 
The  extinction  directions  for  various  parts  of  a  crystal  may  be 
obtained  by  bisecting,  internally  and  externally,  the  angles 
formed  by  joining  any  point  with  the  traces  of  the  optic  axes. 
The  dotted  lines  represent  this  procedure  for  one  point.  In 
similar  manner  the  small  crosses  were  obtained  for  different 


FIG.  360. — Explanation  of  a  biaxial  interference  figure. 

parts  of  the  crystal.  In  the  normal  position,  a  black  cross  will  be 
formed  along  the  vertical  and  horizontal  lines.  On  revolving  the 
section  the  black  cross  disappears.  In  the  45°  position  brush-like 
hyperbolae  are  formed  by  the  darkness  of  different  parts  along  the 
hyperbolae  of  the  figure.  In  the  90°  position  the  cross  is  restored. 

The  number  of  rings  depends  upon  the  strength  of  the  double 
refraction  and  upon  the  thickness,  but  the  distance  between 
the  vertices  of  the  hyperbolae  remains  constant  whatever  the 
thickness. 

The  optical  character  is  determined  by  means  of  a   quartz 


THE  OPTICAL  PROPERTIES  OF  MINERALS  199 

wedge  on  which  is  marked  the  slower  ray  7.  The  quartz  wedge  is 
inserted  in  the  slot,  thin  end  first,  when  the  interference  figure 
shows  hyperbolae.  Then  when  7  is  parallel  to  the  trace  of  the 
axial  plane,  the  ellipses  appear  to  expand  for  positive  crystals 
and  to  contract  for  negative  crystals.  When  7  is  perpendicular 
to  the  trace  of  the  axial  plane,  the  ellipses  contract  for  positive 
crystals  and  expand  for  negative  crystals. 

A  section  of  a  biaxial  crystal  normal  to  an  optic  axis  shows  a 
series  of  concentric  rings  crossed  by  a  dark  bar  which  revolves 
in  an  opposite  direction  from  the  rotation  of  the  stage. 

The  axial  angle  of  a  biaxial  crystal  may  be  measured  by  means 
of.  an  axial  angle  apparatus,  which  is  practically  a  reflection  gonio- 
meter plus  Nicol  prisms.  If  a  suitable  crystal  is  mounted,  so 

that  it  can  be  rotated  around  its  £  direc-  a > 

tion  as  an  axis,  between  horizontal  crossed 
nicols,  arranged  so  that  the  interference 
figure  shows  hyperbolae,  the  apparent 
axial  angle  can  be  determined  by  reading 
the  circle  when  the  vertices  of  the  hyper- 
bolae are  tangent  to  the  cross-wires. 
Figure  361  represents  a  section  of  a  biaxial 

crystal  parallel  at  the  axial  plane.      It  will  relation  between  true  and 

be  seen  that  the  angle  measured  is  not  27,  apparent  axial  angles" 
but  another  angle  which  is  called  2E  and  related  to  it  by  the 
following  equation:  sin  E  =  n^  sin  7. -(rip  being  the  index  of  re- 
fraction in  the  direction  of  the  optic  axis).     The  value  2E  is 
often  recorded,  as  it  is  obtained  directly. 

Another,  but  less  accurate,  method  of  measuring  the  axial 
angle  is  based  upon  the  fact  that  the  distance  apart  of  the 
vertices  (d  in  Fig.  361)  of  the  hyperbolae  of  an  interference  figure 
is  proportional  to  the  value  of  2E.  This  determination  may  be 
made  by  using  a  micrometer  eye-piece  in  the  microscope.  The 
distance  apart  of  the  branches  of  the  hyperbolae  of  a  substance 
with  previously  determined  value  of  2E  is  measured.  This 
determines  the  constant  C  in  the  equation  sin  E  =  d/C.  So 


200        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

for  other  crystals,  if  the  same  microscope  and  combination  of 
lenses  are  used,  the  value  E  may  be  calculated  from  the  measure- 
ment of  d  and  substitution  of  C  in  the  above  formula. 

13.  OPTICAL  PROPERTIES  OF  TWIN-CRYSTALS 

One-half  of  a  twin-crystal  has  the  same  position  with  respect 
to  the  other  half  that  it  would  have  if  it  were  rotated  180°  about  an 
axis  from  its  original  position.  From  this  it  can  be  seen  that  the 
extinction  angles  in  two  halves  of  a  twin-crystal  are  equal,  but 
opposite  in  sign,  if  the  section  is  cut  normal  to  the  twin-plane. 
Thus  in  a  cleavage  of  a  gypsum  twin  the  extinction  directions  are 
inclined  to  each  other.  Consequently  if  examined  between 
crossed  nicols,  one-half  of  such  a  crystal  is  dark,  while  the  other 
half  is  light  (see  Fig.  362).  On  rotation,  the  light  and  dark  parts 
interchange. 


FIG.  362.  Fm.  363.  FIG.  364. 

FIGS.  362-364. — Sections  of  twinned   crystals  examined  between  crossed  nicols. 

A  section  of  a  polysynthetic  twin  such  as  plagioclase  (see  Fig. 
363)  shows  a  series  of  dark  and  light  bands;  the  extinction 
directions  in  alternate  bands  are  parallel. 

Many  orthorhombic  crystals  such  as  aragonite  are  pseudo- 
hexagonal  by  twinning,  and  basal  sections  between  crossed 
nicols  are  divided  into  six  sectors  like  Fig.  364,  opposite  pairs  of 
which  extinguish  together.  Basal  sections  of  aragonite,  like 
Fig.  364,  examined  in  convergent  polarized  light,  show  a  biaxial 
interference  figure  in  each  sector  but  are  arranged  so  that  the  axial 


THE  OPTICAL  PROPERTIES  OF  MINERALS  201 

planes  are  parallel  to  the  outline.     An  optical  examination  reveals 
the  composite  nature  of  many  apparently  simple  crystals. 

14.  ABSORPTION  AND  PLEOCHROISM 

The  color  of  a  transparent  substance  is  due  to  the  residual 
color  of  the  spectrum  left  after  the  substance  has  absorbed  a 
certain  part  of  it.  Many  colored  anisotropic  crystals  have  the 
property  of  absorbing  different  amounts  or  kinds  of  light  in 
different  directions.  Absorption  has  reference  to  the  amount  or 
intensity  of  light  absorbed  and  hence  may  be  tested  in  mono- 
chromatic light,  while  pleochroism  refers  to  the  kind  of  light 
absorbed  and  so  necessarily  must  be  tested  in  white  light. 


FIG.  365. — Dichroscope. 

A  prismatic  crystal  of  epidote  from  the  Sulzbachthal  in  Tyrol 
of  suitable  thickness  will  appear  green  in  a  certain  position,  while 
on  revolving  it  90°  about  its  long  axis  it  will  appear  brown.  Few 
substances  show  such  a  marked  change  as  this;  at  any  rate 
it  is  not  always  possible  to  turn  a  crystal  about  and  look  through 
it  in  various  directions. 

The  determination  of  pleochroism  and  absorption  is  made 
either  by  means  of  one  nicol  of  a  polarizing  microscope,  in  which 
case  minute  crystals  may  be  examined,  or  by  means  of  the  dichro- 
scope  when  large  crystals  are  available. 

A  dichroscope  is  simply  a  piece  of  Iceland  spar  set  in  a  cylin- 
drical frame  that  is  provided  with  a  small  aperture  at  one  end  and 
either  open  or  provided  with  a  lens  at  the  other  end.  Figure  365  is 
a  diagrammatic  representation  of  the  dichroscope.  When  held 
up  to  the  light  the  dichroscope  shows  two  images  of  the  aperture 
side  by  side,  for  the  diameter  of  the  aperture  is  made  so  that 


202        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

the  images  do  not  overlap.  If  a  pleochroic  crystal  is  viewed 
through  the  dichroscope,  two  colored  images  are  seen  simulta- 
neously. The  reason  for  this  is  that  in  a  doubly  refracting  calcite 
crystal  we  have  two  sets  of  light  waves  vibrating  in  planes  at  right 
angles  to  each  other. 

The  color  of  some  crystals,  as  we  have  seen,  varies  with  the 
direction,  but  by  using  a  Nicol  prism  we  may  observe  the  two 
colors  successively,  one  when  the  vibration  plane  of  the  nicol  is 
parallel  to  the  length  of  the  crystal,  and  one  when  perpendicular 
to  the  length  of  the  crystal.  These  colors  are  called  axial  colors. 


FIG.  366.— Biotite  (in  rock  section).  Fio.  367.— Calcite  (cleavage  fragments). 

In  uniaxial  crystals  there  are  two  axial  colors,  hence  the  term 
dichroic  is  used.  In  biaxial  crystals  there  are  three  axial  colors, 
hence  the  term  trichroic. 

In  order  to  determine  the  axial  colors  for  particular  directions 
it  is  necessary  to  ascertain  the  vibration  plane  of  the  lower  nicol. 
For  this  purpose  a  rock-section  containing  biotite  may  be  used. 
Biotite  sections  showing  cleavage  have  very  strong  absorption. 
On  revolving  the  section,  the  biotite  becomes  very  dark  every 
180°.  The  cross-wire  which  is  parallel  to  the  cleavage  traces  of* 
the  biotite  when  it  is  darkest  represents  the  vibration  plane  of  the 
lower  nicol  as  illustrated  in  Fig.  366.  Here  the  arrow  indicates 
the  vibration  plane. 

If  a  rock-section  containing  biotite  is  not  available,  the  test 
may  be  made  by  examining  minute  cleavage  fragments  of  calcite 


THE  OPTICAL  PROPERTIES  OF  MINERALS 


203 


obtained  by  pounding  to  a  coarse  powder  almost  any  kind  of  cal- 
cite.  If  mounted  in  oil  of  cloves  or  Canada  balsam,  the  calcite 
rhombs  have  a  marked  relief  when  their  long  diagonals  are 
parallel  to  the  vibration  plane  of  the  lower  nicol,  and  but  slight 
relief  when  their  short  diagonals  are  parallel  to  this  direction  as 
represented  in  Fig.  367.  The-  vibration  direction  of  the  lower 
nicol  is  indicated  by  the  arrow. 

Fragments  of  fibrous  tourmaline  may  also  be  used  for  the  same 
purpose.  The  prismatic  fragments  appear  dark  when  their 
length  is  perpendicular  to  the  vibration  plane  of  the  lower  nicol. 
(See  Fig.  330,  p.  173.) 


FIG.  368.  FIG.  369.  FIG.  370. 

FIGS.  368-370. — Pleochroism  of  glaucophane. 

The  trichroism  of  a  biaxial  crystal  is  beautifully  illustrated  by 
the  soda  amphibole,  glaucophane,  as  seen  in  thin  rock-sections 
under  the  microscope.  Three  kinds  of  cross-sections  may  be 
distinguished  as  follows:  pseudo-hexagonal  (Fig.  368),  stout 
rectangular  (Fig.  369),  and  thin  with  oblique  ends  (Fig.  370). 
These  sections  are  respectively  almost  perpendicular  to  the  c-,  a-, 
and  6-axes.  These  sections  when  rotated  on  the  stage  of  the 
microscope  show  respectively  the  following  pairs  of  colors:  neu- 
tral and  violet,  blue  and  violet,  blue  and  neutral.  It  will  be  seen 
that  the  color  for  the  a-axis  is  the  neutral  tint,  for  the  mineral 
has  this  color  when  the  a-axis  is  parallel  to  the  vibration  plane  of 
the  nicol  (represented  by  the  arrow).  Similarly  the  violet  is  the 


204        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

color  for  the  6-axis,  and  blue  for  the  c-axis.     The  three  axial  colors 
may  be  combined  in  an  axial  cross. 

In  glaucophane  it  has  been  found  that  b  =  0,  and  a  (almost)  = 
a,  and  c  (almost)  =  7.  The  absorption  may  be  indicated  by  the 
following:  7  >/?>«.  This  is  called  the  absorption  scheme  and 
means  that  more  light  is  absorbed  in  the  7  or  c-direction  than  in 
the  ]8  or  6-direction,  and  more  in  the  /3  or  6-direction  than  in  the 
a  or  a-direction.  Sometimes  Gothic  letters  are  used  instead 
of  a,  /?,  and  7. 

15.  SUGGESTED  OUTLINE  OF  TESTS  TO  ILLUSTRATE  THE  OPTI- 
CAL PROPERTIES  OF  MINERALS 

The  following  outline  will  serve  as  an  introduction  to  the  study  of  crystal 
optics.  Most  of  the  slides  are  made  from  cleavage  flakes  and  fragments, 
but  thin  sections  are  necessary  for  some  of  the  examples.  The  fragments 
are  produced  by  pounding  (not  grinding)  small  chips  of  the  mineral  to  coarse 
powder  on  an  anvil  or  in  an  agate  mortar.  The  largest  fragments  that  will 
pass  through  a  100-mesh  sieve  are  selected. 

For  temporary  slides,  clove  oil  is  a  convenient  mounting  medium.  Per- 
manent slides  may  be  made  by  using  a  solution  of  Canada  balsam  in  xylol. 
The  xylol  gradually  evaporates. 

On  account  of  cleavage  and  structure  a  great  many  mineral  fragments 
have  a  more  or  less  characteristic  shape.     Those  without  cleavage  are 
irregular  (see  Fig.  377). 
Form. 

Euhedral.     Calamine  crystal.     (Measure  angles  and  determine  faces). 

Subhedral—  dolomite  in  sedimentary  dolomitic  limestone. 

Anhedral — quartz  in  sandstone. 
Cleavage  fragments — (See  Figs.  371-376.) 
•  Triangular  cleavage  fragments — fluorite.     (Fig.  371.) 

Rectangular  cleavage  fragments — anhydrite.     (Fig.  372.) 

Rhombic  cleavage  fragments — calcite.     (Fig.  373.) 

Prismatic  cleavage  fragments — tremolite.     (Fig.  374.) 

Acicular  cleavage  fragments — wollastonite.     (Fig.  375.) 

Platy  cleavage  fragments  (not  previously  included)— orthoclase.  (Fig.  376.) 

Irregular  fragments — quartz.     (Fig.  377.) 
Inclusions. 

Regularly  arranged — labradorite,  phlogopite. 
Intergrowth — perthite  (microcline  with  albite). 
Alteration — olivine  to  antigorite. 


THE  OPTICAL  PROPERTIES  OF  MINERALS 

Index  of  Refraction  and  Relief. 
High  relief,  n< clove  oil — fluorite. 
Low  relief,  n<  (about  =)  clove  oil — orthoclase. 
High  relief,  n> clove  oil — anhydrite. 


205 


FIG.  377. 
FIGS.  371-377. — Cleavage  fragments  as  observed  under  compound  microscope. 

Pleochroism  and  Absorption. 

Pink  to  red — pale  blue  to  indigo — tourmaline. 


206        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Blue  to  purple — glaucophane. 

Pink  to  pale  green — hypersthene. 
Relief  varies  with  the  direction — calcite. 

(High  relief  when  long  diagonal  of  rhomb  is  parallel  to  the  vibration 

plane  of  the  lower  nicol.) 
Isotropic. 

Amorphous — opal,  volcanic  glass. 

Isometric — fluorite. 

Anisotropic — anhydrite,  quartz,  apatite,  etc. 
Interference  Colors. 

Due  to  thin  films — cleavage  cracks  in  gypsum,  calcite,  or  fluorite. 

Effect  of  thickness — Gypsum  wedge  (made  by  shaving  down  a  cleavage 

flake  of  gypsum.) 

{Low-order  colors — apatite. 
Bright  colors — anhydrite. 
High-order  colors — calcite. 
Effect  of  orientation — Quartz  in  sandstone. 
Extinction. 

Parallel — wollastonite. 
Symmetrical — calcite,  dolomite. 

Oblique,  small  extinction  angle — hornblende,  tremolite. 
Oblique,  large  extinction  angle— augite,  diopside. 
Elongation. 

Parallel  to  faster  ray — stilbite. 
Parallel  to  slower  ray — wollastonite. 
Aggregate  Polarization — chrysocolla. 
Spherulitic  Structure — chalcedony. 
Optical  Anomalies. 

Amorphous,  but  doubly  refracting — cellophane. 
Isometric  mineral  with  double  refraction — leucite. 
Anomalous  interference  colors — vesuvianite,  chlorite. 
Twinning. 

Simple — gypsum . 
Polysynthetic — plagioclase. 
Crossed — microcline. 
Interference  Figures. 
Uniaxial  positive. 

Brucite,  cleavage;  quartz  (basal  section). 
Uniaxial  negative. 

Calcite,  basal  parting;  wulfenite,  tabular  crystal. 
Biaxial  positive,  small  axial  angle. 
Chlorite,  cleavage. 


THE  OPTICAL  PROPERTIES  OF  MINERALS  207 

Biaxial  positive,  large  axial  angle. 

Topaz,  cleavage. 
Biaxial  negative,  small  axial  angle. 

Phlogopite,  cleavage. 
Biaxial  negative,  large  axial  angle. 

Muscovite,  cleavage. 
Biaxial,  normal  to  optic  axis,  shows  axial  bar. 

Epidote,  (001)  cleavage;  diopside  (001)  parting. 

Optical  Orientation. — Muscovite  crystal  (determine  position  of  a,  /3,  and 
7  with  respect  to  crystallographic  axes  a,  b,  and  c). 

16.  LIST  OF  MINERALS  ARRANGED  ACCORDING  TO  INDICES  OF 

REFRACTION 

An  arrow  after  a  mineral  name  indicates  that  the  mineral  is 
doubly  refracting.  If  the  arrow  points  downward,  the  lower 
value  (na)  is  the  one  given,  and  the  highest  value  (nT)  may  be 
found  in  a  place  further  on  in  the  list.  Other  possible  values 
lie  between  these  two  extremes. 

The  fact  that  no  arrow  is  placed  after  a  mineral  name  means 
that  the  mineral  is  either  optically  isotropic  (amorphous  or  iso- 
metric) or  has  such  a  weak  double  refraction  (less  than  0.01) 
that  the  extreme  values  are  included  within  a  single  division. 
Some  amorphous  and  isometric  minerals  are  variable  in  com- 
position and  so  may  be  found  in  two  or  more  divisions. 

<  1 . 45  CALCITE  J,  Ulexite  J, 

Cryolite  Carnallite  ]  1.51 

FLUORITE  Chabazite  Adularia  j 

Ice  Cristobalite  Chalcanthite  J, 

Nitratine  J  Natrolite  |  ORTHOCLASE  J 

1 . 45  Sodalite  Ulexite  j 
OPAL  1.49  1.52 

1 . 46  Heulandite  Adularia  f 
Carnallite  j  Kainite  j  GYPSUM 
CHRYSOCOLLA  J,  Stilbite  Hydromagnesite  J 

1 . 47  Sylvite  Kainite  | 
Natrolite  j  1 . 50  Magnesite  J, 
Tridymite  DOLOMITE-J,  Microcline 

1-48  Lazurite  ORTHOCLASE  | 

Analcite  Leucite  Strontianite 


208        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Witherite  J, 
1.53 

Albite 

Apophyllite 

Aragonite  j 

CHALCEDONY  J, 

Gibbsite  j 

Nepheline  J, 
1.54 

Chalcanthite  j 

CHALCEDONY  | 

Chrysotile  | 

HALITE 

Hydromagnesite  f 

Oligoclase 

Nepheline  f 

QUARTZ  J, 

TALC  | 
1.55 

Andesine 

Chrysotile  f 

Gibbsite  f 

Halloysite 

QUARTZ | 

Scapolite  J, 
1.56 

ANTIGORITE  j 

Beryl 

Brucite  J, 

CHRYSOCOLLAt 

Kaolinite 

Labradorite 

Lepidolite  J, 

MUSCOVITE  | 

Phlogopite  J, 

Sericite  J, 
1.57 

ANHYDRITE  J, 

Anorthite  J, 

ANTIGORITE  | 

BIOTITE  | 

Bytownite 


CLIACHITE 

Pyrophyllite 

Vivianite  j 
1.58 

Anorthite  f 

Brucite  f 

CHLORITE 

Colemanite  \, 

COLLOPHANE 

Garnierite 

Nitratine  j 

Pyrophyllite 

TALCf 
1.59 

CHLORITE 

COLLOPHANE 

Lepidolite  f 

MUSCOVITE  | 

Scapolite  f 

Sericite  | 
1.60 

Chondrodite  J, 

COLLOPHANE 

Rhodochrosite  | 

Phlogopite  | 
1.61 

ANHYDRITE  | 

CALAMINE 

Colemanite  j 

COLLOPHANE 

Dahllite 

Prehnite 

SMITHSONITE  | 

Topaz  | 

Tremolite  j 

Turquois  | 
1.62 

Celestite 

COLLOPHANE 

Dahllite  | 

Datolite  | 

Glaucophane  | 


Topaz  | 
Vivianite  | 
Wollastonite  I 

1.63 

Andalusite 
Anthophyllite  \, 
BARITE  I 
BIOTITE  | 
CALAMINE  | 
Chondrodite  | 
Celestite  | 
Forsterite  J, 
HORNBLENDE 
SIDER1TE  | 
TOURMALINE 
Tremolite  f 
Wollastonite  T 

1.64 

APATITE 
BARITE  t 
Glaucophane  f 
Prehnite  t 

1.65 

Anthophyllite  j 
CALCITE  | 
HORNBLENDE 
MALACHITE  [ 
Spodumene  J, 
TOURMALINE  t 
Turquois  | 

1.66 

Datolite  f 
Enstatite  J, 
OLIVINE  I 
Sillimanite  t 

1.67 
Axinite 
Diopside  J, 
Enstatite  | 
Forsterite  f 
Spodumene  f 
Strontianite  f 
Witherite 


THE  OPTICAL  PROPERTIES  OF  MINERALS 


209 


.68 

1.73 

Hausmannite 

Aragonite  j 

Augite  f 

HEMATITE 

DOLOMITE  t 

Brochantite  J, 

Jarosite 

Sillimanite  f 

EPIDOTE  | 

LIMONITE 

.69 

Grossularite 

MALACHITE  | 

Hypersthene  J, 

Rhodonite  | 

Mimetite 

OLIVINE  t 

Staurolite  j 

Pitchblende 

Willemite  j 

1.74 

Polybasite 

.70 

(Methylene  lodid) 

Pyrargyrite 

Diopside  f 

Anglesite 

Pyromorphite 

Hypersthene  f 

Azurite 

Rhodochrosite  | 

Willemite  | 

Brochantite  f 

Rutile 

.71 

Carnotite 

Scheelite 

Augite  J, 

CASSITERITE 

SIDERITE  I 

Clinozoisite  J, 

Cerargyrite 

SMITHSONITE 

Kyanite  J, 

CERUSSITE 

SPHALERITE 

Magnesite  f 

CHROMITE 

Staurolite  f 

.72 

CINNABAR 

SULFUR 

Clinozoisite  f 

CORUNDUM 

Titanite 

Kyanite  f 

Cuprite 

Turyite 

Rhodonite  J, 

Diamond 

Vanadinite 

Spinel 

EPIDOTE  t 

Wulfenite 

Vesuvianite 

Goethite 

Zircon 

GARNET 

14 


MINERALS 

1.  ELEMENTS. 

2.  SULFIDS. 

3.  SULFO-SALTS. 

4.  HALOIDS. 

5.  OXIDS. 

6.  ALUMINATES,  FERRITES,  ETC. 

7.  HYDROXIDS. 

8.  CARBONATES. 

9.  PHOSPHATES,  NITRATES,  BORATES,  ETC 

10.  SULFATES. 

11.  TUNGSTATES  AND  MOLYBDATES. 

12.  SILICATES. 

MINERALOIDS 
(GLASS  AND  HYDROCARBONS) 


210 


PART  II 

THE  DESCRIPTION  OF  IMPORTANT  MINERALS  AND 
MINERALOIDS 

Introductory 

About  a  thousand  or  so  distinct  kinds  of  minerals  or  mineral 
species  are  recognized  by  the  mineralogist.  Most  of  these  are 
very  rare,  many  of  them  being  found  only  at  the  single,  original 
locality  in  which  they  were  discovered.  In  this  book  175  minerals 
are  considered.  These  include  all  the  common  minerals,  most 
of  those  of  economic  importance,  and  a  few  others  which  are 
added  so  as  to  give  the  student  a  comprehensive  view  of  the 
mineral  kingdom  as  a  whole.  The  student  may  occasionally 
encounter  a  mineral  not  included  in  the  list  of  175  and  the  larger 
reference  books  such  as  Dana's  System  and  Hintze's  Handbuch 
must  be  consulted.  There  is  also  a  possibility  of  finding  a  new 
mineral,  but  the  chance  is  very  remote  for  bona  fide  new  minerals 
are  being  found  and  described  at  the  rate  of  only  about  ten  or  so 
a  year  by  mineralogists  the  world  over. 

By  mineral  species  we  mean  all  specimens  with  essentially  the 
same  chemical  composition  (some  variation  must  be  allowed  as 
stated  on  page  13)  and  the  same  crystal  form.  Each  mineral 
species  has  a  distinctive  name  in  addition  to  the  chemical  name 
of  the  substance  of  which  it  is  composed.  The  name  connotes 
certain  physical  properties  in  addition  to  chemical  composition, 
for  polymorphous  modifications  and  amorphous  equivalents  are 
recognized  as  distinct  minerals. 

Most  mineral  names  end  in  -ite.  This  custom  has  its  origin  in 
the  practice  of  the  Greeks  and  Romans  of  adding  the  suffix 
-ites  or  -itis  (originally  from  the  Greek  lithos,  a  stone)  to  a  word 
which  was  descriptive  of  the  mineral,  or  indicated  its  use  or  the 
locality  in  which  it  was  found.  Other  endings  used  especially 

an 


212        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

by  Haiiy  are  ~ane,  -ase,  -ene,  -ine,  -ose,  and  -ote.  Names  in  use 
before  the  custom  of  using  the  termination  ite  was  adopted  are 
quartz,  opal,  topaz,  garnet,  mica,  diamond,  galena,  beryl,  gyp- 
sum, zircon,  and  hornblende.  It  seems  fortunate  that  we  have 
some  variety  in  our  mineralogical  nomenclature  especially  since 
the  termination  -ite  has  been  used  for  rock  names  (granite, 
syenite,  etc.)  and  for  artificial  compounds  (aloxite,  quercite,  etc.). 

Dana  in  1837  used  a  binomial  nomenclature  for  minerals  like 
that  at  present  used  for  plants  and  animals.  Thus  the  genus 
Baralus  included  Baralus  ponderosus  (barite),  E.  prismaticus 
(celestite),  B.fusilis  (witherite),  andB.  rubefaciens  (strontianite). 
The  classification  used  then  was  based  upon  external  characters 
or  physical  properties.  This  gradually  gave  way  to  the  chemical 
classification  of  Berzelius  and  the  Swedish  chemists. 

The  modern  chemical  classification  is  one  in  which  the  minerals 
are  arranged  according  to  the  acid  radical.  The  principal 
classes  of  minerals  are:  elements,  sulfids,  sulfo-salts,  haloids, 
oxids,  hydroxids,  carbonates,  phosphates,  nitrates,  borates, 
sulfates,  tungstates,  and  silicates.  Within  each  of  these  divi- 
sions the  minerals  are  arranged  as  far  as  possible  in  isomorphous 
groups.  Thus  calcite  (CaCO3),  dolomite  (CaMg[CO3]2),  magne- 
site  (MgCO3),  siderite  (FeCO3),  rhodochrosite  (MnCO3),  and 
smithsonite  (ZnCO3)  are  included  in  the  calcite  group  of  rhom- 
bohedral  carbonates,  for  they  crystallize  in  the  hexagonal  system 
and  have  rhombohedral  cleavage  and  similar  optical  properties. 
Aragonite  (CaC03)  and  cerussite  (PbCO3)  together  with  the 
carbonates  of  strontium  and  barium  constitute  another  group  of 
orthorhombic  carbonates,  while  malachite  [Cu2(OH)2CO3]  and 
azurite  [Cu(OH)2(CO3)2]  and  hydromagnesite  [Mg4(OH)2(CO3)3- 
3H2O]  must  be  considered  separately  because  they  are  basic  carbon- 
ates unlike  in  crystal  form  and  physical  properties. 

Certain  naturally  occurring  homogeneous  substances  not 
definite  enough  in  chemical  composition  to  be  called  minerals  are 
considered  under  the  term  mineraloid.  They  include  volcanic 
glass  and  the  hydrocarbons. 


1.  ELEMENTS 

A.  Non-metals 

Diamond  C 

GRAPHITE  C 

SULFUR  S 

B.  Metals 

GOLD  Au 

SILVER  Ag 

COPPER  Cu 

Platinum  Pt 

Iron  Fe 

Of  the  eighty  or  more  known  elements  only  about  twenty 
occur  uncombined  as  minerals,  if  we  leave  out  of  consideration  the 
free  gases  of  the  atmosphere.  The  elements  occurring  free  and 
uncombined  are:  carbon,  sulphur,  selenium,  tellurium,  phos- 
phorus, arsenic,  antimony,  bismuth  mercury,  copper,  silver,  gold, 
lead,  iron,  palladium,  iridium,  osmium,  tantalum,  and  tin.  From 
a  chemical  standpoint  the  elements  may  be  divided  into 
two  classes :  the  metals  and  the  non-metals.  The  metals  include 
such  elements  as  copper,  silver,  gold,  lead,  iron,  and  platinum. 
Some  of  these  occur  as  alloys  such  as  electrum  (Au,Ag),  amalgam 
(Ag,Hg),  nickel-iron  (Fe,Ni),  and  iridosmine,  (Ir,Os).  The  non- 
metals  include  such  elements  as  oxygen,  hydrogen,  nitrogen, 
phosphorus,  carbon,  and  sulfur.  Arsenic,  antimony,  and  bismuth 
are  intermediate  in  their  properties  between  metals  and  non-metals 
and  are  usually  called  semi-metals  or  metalloids. 

Diamond,  C 

Form.  Diamond  is  practically  always  found  in  small  loose 
crystals  with  rounded  faces  and  curved  edges.  It  crystallizes 
in  the  isometric  system,  probably  in  the  hexoctahedral  class. 
Though  many  of  the  isometric  forms  have  been  observed,  the 

213 


214        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

only  common  well-defined  one  is  the  octahedron.  Figure  378 
represents  a  typical  crystal  with  grooved  edges  and  triangular 
markings.  Spinel  twins  of  diamond  are  rather  common. 

The  internal  structure  of  diamond  determined  by  X-ray  analy- 
sis is  like  that  of  sphalerite  (Fig.  298,  p.  144)  except  that  all  the 
atoms  are  carbon  atoms. 

Cleavage.     Perfect  octahedral.     This  fact  enables  the  dia- 
mond-cutter to  save  considerable  work. 
H.  =  10  (the  hardest  known  substance). 

Color.  Diamond  is  usually  colorless  or  faintly  colored,  though 
brilliantly  colored  blue,  green,  and  red  stones  are  known.  One 
variety  known  as  carbonado  is  black  and  opaque. 

Luster.  The  luster  is  the  brilliant  luster 
known  as  adamantine.  The  rough  uncut 
crystals  have  a  peculiar  greasy  appearance. 
Optical  Properties.  The  index  of  re- 
fraction is  very  high  (n  =  2.4 175  for  sodium 
light)  which  accounts  for  its  brilliancy. 
The  "fire"  of  the  diamond  is  accounted 
for  by  its  strong  dispersion;  the  index  of 
refraction  for  the  red  end  of  the  spectrum 

FlQ'    3  7  cr  ltd  * a  m  °  n  d  is  2'402'  while  f °r  the  violet  end  {i  is  2'465* 
Diamonds  are  transparent  to  X-rays,  while 

imitations  are  opaque. 

Chemical  Composition.  Pure  carbon.  Upon  heating  the 
diamond  in  an  atmosphere  of  oxygen  it  is  converted  into  C02- 

Blowpipe  Tests.     Infusible.     Insoluble  in  acids. 

Distinguishing  Features.  Diamond  is  distinguished  from 
similar  minerals  by  its  superior  hardness,  its  adamantine  luster, 
and  its  comparatively  high  specific  gravity. 

The  peculiar  rounded  crystals  with  an  apparently  oiled  sur- 
face are  unlike  those  of  any  other  mineral. 

Uses.  On  account  of  its  great  hardness,  brilliancy,  and  rarity, 
diamond  stands  as  the  gem  mineral,  par  excellence.  Among  the 
famous  historic  diamonds  are  the  Kohinoor,  186  carats;  the 


ELEMENTS  215 

Regent,  137  carats;  the  Star  of  the  South,  254  carats;  the  Im- 
perial, 457  carats;  and  the  Excelsior,  969  carats.  Of  colored 
diamonds  the  most  famous  are  the  Tiffany  (orange-yellow), 
the  Hope  (greenish-blue),  the  Dresden  (bluish-green),  and  the 
Paul  I.  (ruby-red).  The  largest  diamond  on  record  is  the  Cul- 
linan  (since  named  the  Star  of  Africa)  found  in  1905  at  the  Pre- 
mier mine  in  the  Transvaal.  This  diamond  weighed  3106  metric 
carats  (621.2  grams  or  about  1^  pounds  avoirdupois). 

Diamonds  are  also  used  as  an  abrasive  in  cutting  and  polishing 
precious  stones,  glass,  and  other  materials.  The  center  of  the 
diamond  cutting  industry  is  Amsterdam. 

Several  mines  within  an  area  of  ten  square  miles  at  Kimberley, 
South  Africa,  have  furnished  the  world's  principal  supply  of 
diamonds  since  their  discovery  in  1867. 

A  black,  opaque,  non-cleavable  variety  of  diamond  is  used  for 
diamond  drills.  It  is  found  only  in  Bahia,  Brazil,  and  is  known 
as  carbonado  or  "  black  diamond." 

Occurrence.  1.  In  volcanic  necks  and  dikes  of  a  rock  known  as 
kimberlite  (locally  called  "blue-ground").  Kimberlite  is  an 
altered  peridotite  composed  of  fragments  of  pyrope,  pyroxene, 
biotite,  olivine,  etc.,  in  a  matrix  of  serpentine.  The  origin  of  the 
diamond  is  in  doubt,  but  many  believe  it  to  be  of  igneous 
origin. 

Diamonds  have  recently  been  found  in  a  peridotite  dike  in 
Pike  County,  Arkansas.  The  stones  are  small  in  size  but  of  very 
good  quality. 

2.  In    alluvial     deposits    associated    with    heavy    minerals. 
Among  these  localities  may  be  mentioned  southern  India,  (where 
diamonds    were  first  found),   the  states  of  Bahia  and   Minas 
Geraes,  Brazil  (one  locality  is  known  as  Diamantina),  and  scat- 
tered localities  throughout  the  United  States.     In  the  Great 
Lakes  region,  diamonds  are  found  in  glacial  drift.     In  California 
small  diamonds  have  frequently  been  found  in  sluice-boxes  along 
with  gold. 

3.  In  the  chromite  of  serpentinized  dunite  in  the  Tulameen 


216        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

district,  British  Columbia,  minute  diamonds  have  recently  been 
found. 

4.  In  meteorites  from  Canon  Diablo  (Arizona),  minute  dia- 
monds have  been  found.  Moissanite,  SiC,  the  same  as  the  arti- 
ficial carborundum,  is  also  found  there.  A  peridotite  meteorite 
which  fell  at  Novo-Urei,  Russia,  in  1886  also  contained  diamonds. 

Moissan  obtained  small  diamonds  by  cooling  in  water  a  block  of 
soft  iron  saturated  with  carbon. 

GRAPHITE,   C 

Form.  Graphite  occurs  occasionally  in  six-sided  tabular 
crystals,  but  more  often  in  foliated  masses,  minute  disseminated 
scales,  or  earthy  lumps. 

Cleavage.     Cleavage  in  one  direction. 

H  =  1  to  2.  Sp.  gr.  2.1  ±. 

Color.  Dark  gray  to  black.  Luster,  metallic.  Streak,  gray. 
Opaque  even  in  thinnest  fragments.  Sectile. 

Chemical  Composition.  Graphite  is  a  modification  of  carbon. 
It  grades  from  pure  carbon  to  earthy  varieties  which  yield  a  large 
amount  of  ash  on  combustion. 

Blowpipe  Tests.  The  tests  for  graphite  are  largely  negative  on 
account  of  its  refractory  nature.  Infusible.  Insoluble  in  acids. 

Distinguishing  Features.  Graphite  resembles  molybdenite 
but  is  distinguished  by  its  lower  specific  gravity,  negative  NaPO3 
bead  test,  negative  sulfid  test,  and  difference  in  streak  on  glazed 
porcelain  or  glazed  paper.  (Molybdenite  has  a  greenish  gray 
streak.) 

It  is  distinguished  from  hematite  by  its  streak  and  lower 
specific  gravity,  from  magnetite  by  its  inferior  hardness  and 
failure  to  be  attracted  by  a  magnet,  and  from  hydrocarbons  by 
its  non-volatility  in  the  closed  tube. 

Uses.  Graphite  is  used  in  the  manufacture  of  lead  pencils 
(varying  hardness  is  due  to  admixed  clay),  lubricants  for  ma- 
chinery, refractory  crucibles,  and  electrical  supplies.  Austria 
and  Ceylon  are  the  chief  producers. 


ELEMENTS  217 

Artificial  graphite  is  now  made  from  anthracite  in  electric 
furnaces  at  Niagara  Falls. 

Occurrence.  1.  In  crystalline  limestones,  doubtless  formed  by 
the  recrystallization  of  the  organic  matter  of  sedimentary  lime- 
stones. Franklin,  New  Jersey. 

2.  In  schists  and  gneisses, .  of  ten  as  an  essential  constituent. 
Hague,  New  York. 

3.  In  veins,  in  granites  and  granulites,  which  proves  that  its 
origin  may  be  independent  of  previous  life.     Ceylon. 

4.  In  coal-beds,  the  coal  is  often  converted  into  graphite  near 
the  contact  with  igneous  intrusions. 

5.  In   meteorites.     Paramorphs   of  graphite   after   diamond, 
called    cliftonite,    are   also   found   in   meteorites.     (On  heating 
diamond  out  of  contact  with  air  it  is  converted  into  graphite) . 

SULFUR,  S 

Form.  Sulfur  occurs  in  crystals,  incrustations,  dissemina- 
tions, and  compact  masses.  The  crystals  are 
good  examples  of  the  orthorhombic  system. 
Usual  forms:  p{lll},  c{001),  n{011|,  and 
s  { 1 13 } .  The  habit  is  usually  pyramidal,  with 
{111}  as  the  dominant  form.  Interfacial 
angles:  pp(lll  :  ill)  =  94°  52',  cp(001  :  111) 
=  71°  40',cs(001  :  113)  =  45°  10',  cn(001  : 
Oil)  =  62°  17'.  Figure  379  is  the  common 
type  of  crystal.  FIG.  379.— Sulfur 

H.  =  IK  to  2%.     Sp.  gr.  2.07  ±.  crystah 

Color.     Yellow,  sometimes  with  orange,  brown,  or  green  tinge. 

Luster.     Resinous  to  adamantine. 

Optical  Properties.  w7(?.24)  -  w0(1.95)  =  0.29.  Fragments 
are  irregular  with  high-order  interference  colors. 

Chemical  Composition.  Sulfur,  often  with  such  impurities  as 
clay  and  asphaltum.  Some  varieties  contain  selenium. 

Blowpipe  Tests.  Fuses  easily  (114°  C.)  and  burns  with  a  blue 
flame,  giving  off  sulfur  dioxid.  If  impure,  a  residue  is  left. 


218         INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Insoluble  in  acids.  Soluble  in  carbon  bisulfid  and  on  evapora- 
tion minute  crystals  are  formed. 

Distinguishing  Features.  Sulfur  is  not  apt  to  be  confused 
with  any  other  mineral.  It  may,  however,  be  overlooked  when 
occurring  as  an  impregnation  of  tuff  or  clay,  as  the  color  is  apt  to 
be  gray  instead  of  yellow.  This  material  will  burn  and  leave  a 
residue. 

Uses.  Sulfur  is  used  in  the  manufacture  of  gunpowder, 
matches,  for  vulcanizing  rubber,  and  for  the  production  of  sulfur 
dioxid,  which  is  used  in  paper  manufacturing  and  bleaching.  It 
was  formerly  used  in  the  manufacture  of  sulfuric  acid,  but  that  is 
now  made  from  pyrite.  The  island  of  Sicily  and  southwestern 
Louisiana  are  the  principal  sources  of  sulfur.  At  the  latter  local- 
ity the  sulfur  is  obtained  by  dissolving  it  in  superheated  steam 
and  pumping  the  solution  to  the  surface. 

Occurrence.  1.  As  a  sublimate  in  the  crevices  around  vol- 
canoes (called  solfataras)  or  as  an  impregnation  of  tuffs.  Formed 
by  the  incomplete  oxidation  of  hydrogen  sulfid.  2H2S  +  02  = 
2  S  +  2H20. 

2.  In  sedimentary  rocks   (limestones,   travertine,   and  marl) 
with  gypsum,  and  occasionally  with  celestite.     Girgcnti,  Sicily. 

3.  As   a   decomposition   product   of  sulfids   such   as   galena, 
stibnite,  sphalerite,  and  pyrite. 

4.  Formed    by   some    bacteria   and    algae   in   sulfate-bearing 
waters. 

Sulfur  furnishes  one  of  the  best  examples  of  polymorphism. 
The  orthorhombic  modification,  known  as  a-sulfur,  is  the  one 
that  occurs  so  extensively  in  nature.  The  monoclinic  modifica- 
tion (0-sulfur)  formed  when  sulfur  solidifies  from  fusion  has  been 
noted  in  nature  several  times,  but  it  rapidly  changes  to  the 
orthorhombic  form  on  standing. 

GOLD,  AM 

Form.  Though  usually  finely  disseminated  through  contain- 
ing rock  and  only  apparent  on  assaying,  gold  also  occurs  in  rolled 


ELEMENTS 


219 


grains  and  scales,  occasionally  in  large  nuggets,  and  rarely  in 
crystals  and  imperfect  crystal  aggregates.  Like  many  of  the 
metals,  gold  crystallizes  in  the  isometric  system,  the  octahedron 
being  the  only  common  form.  Sheets  of  gold  (leaf-gold)  with 
raised  triangular  markings  are  not  uncommon. 

H.  =  2J>£  to  3.     Sp.  gr.  =  15  to  19  (according  to  purity). 

Pure  gold  has  a  specific  gravity  of  19.3. 

Color.  Deep  to  pale  yellow.  The  pale  yellow  variety 
containing  over  20  per  cent,  silver  is  known  as  electrum.  Luster, 
metallic.  Very  malleable. 

Chemical  Composition.  Gold,  always  alloyed  with  more  or 
less  silver,  sometimes  also  with  Bi,  Cu,  Fe,  Pd,  and  Rh.  The  purity 
of  gold  is  expressed  by  the  amount  of  gold  in  1000  parts.  For 
example,  gold  with  13  per  cent,  silver  has  a  fineness  of  870.  The 
following  analyses  give  an  idea  of  the  variation  in  the  chemical 
composition. 


Au 

Ag 

Misc. 

Sp.  Gr. 

South  Australia.  ...... 
Urals  

93.5 

87.4 

6.5 
12.1 

Cu  =  0.1 

18.8 
17.4 

Peru  
Verespatak 

79.9 
66  4 

20.1 
33  2 

SiO2  =04 

16.6 
15  0 

Blowpipe  Tests.  Fusible  at  3.  With  mercury  forms  an  amal- 
gam. 

Soluble  in  aqua  regia;  the  silver-bearing  varieties  leave  a 
residue  of  AgCl. 

Distinguishing  Features.  Pyrite  and  chalcopyrite  are  some- 
times mistaken  for  gold  but  they  are  easily  distinguished  by  their 
brittleness,  lower  specific  gravity,  and  by  the  fact  that  they  are 
soluble  in  nitric  acid.  Yellow,  decomposed  scales  of  mica  are 
even  more  like  gold  in  appearance,  but  a  careful  examination 
reveals  their  true  nature. 

Uses.  Native  gold  is  the  source  of  most  of  the  gold  of  com- 
merce, but  some  is  derived  from  the  gold  tellurid  (Calaverite). 


220        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

In  order  of  their  production  the  countries  are:  Transvaal, 
United  States,  Australia,  Russia,  and  Canada.  The  States  in 
order  are:  California,  Colorado,  Alaska,  Nevada,  and  South 
Dakota. 

Occurrence.  1.  In  quartz  veins  along  with  pyrite,  chalco- 
pyrite,  galena,  sphalerite,  arsenopyrite,  etc. 

2.  In  placers  along  streams  (ancient  river-channels  in  some 
cases)  associated  with  heavy  minerals  such  as  magnetite,  ilmen- 
ite,    garnet,    zircon,    platinum,    etc.     Prominent    localities    are 
Alaska,  California,  Brazil,  Colombia,  Urals,  and  Australia. 

3.  In  the  quartzite  conglomerate   (''banket")  on  the  Rand, 
Transvaal,  South  Africa.     The  origin  of  the  gold  is  in  doubt. 

4.  In  the  gossan  or  oxidized  zone  as  a  secondary  mineral 
or  mechanically  released.     At  Cripple  Creek,  gold  pseudomor- 
phous  after  calaverite  is  found  in  the  oxidized  zone. 

SILVER,  Ag 

Form.  The  characteristic  occurrences  of  silver  are  in  wire  form , 
thin  sheets,  skeleton  crystals,  dendritic  groups,  and  masses. 
The  cube  is  the  only  common  crystal  form,  but  the  crystals  are 
very  small. 

H.  =  %.  Sp.  gr.  10.5+. 

Color.  Tin-white  to  pale  yellow,  but  is  usually  dull  and  tarnished. 
Luster,  metallic.  Malleable. 

Chemical  Composition.  Silver,  often  with  some  gold  and 
copper. 

Blowpipe  Tests.     Easily  fusible  (2)  to  a  malleable  button. 

Soluble  in  HNO3.  HC1  gives  a  white  precipitate  (AgCl), 
which  turns  violet  on  standing  and  is  soluble  in  .NH4OH. 

Distinguishing  Features.  Silver  resembles  some  of  the  other 
native  metals  but  is  easily  distinguished  by  the  color  of  a  freshly 
cut  surface. 

Uses.  Native  silver  is  the  source  of  some  silver,  although  most 
of  the  supply  is  derived  from  the  sulfid  and  sulfo-salts.  The 
Cobalt  (Ontario)  district  now  furnishes  about  one-tenth  of  the 


ELEMENTS  221 

world's  supply  of  silver,  the  silver  being  largely  in  the  form  of  the 
native  metal. 

Occurrence.  1.  In  veins  associated  with  cobalt  and  nickel 
minerals.  Cobalt,  Ontario,  is  a  prominent  locality. 

2.  In  veins  with  argentite,  pyrargyrite,  polybasite,  stephanite, 
etc.,  and  usually  the  last  mineral  to  be  formed  and  probably  by 
ascending  solutions. 

3.  In  the  gossan  or  oxidized  zone,  often  associated  with  cerussite 
and   limonite,   and   doubtless   formed   by  descending  solutions. 
Leadville,  Colorado,  and  the  Coeur  d'Alene  district  in  Idaho  are 
prominent  localities. 

In  contrast  with  gold,  silver  is  practically  never  found  in 
placers. 

COPPER,  Cu 

Form.  Copper  is  found  in  small  disseminated  grains,  in  sheets, 
and  occasionally  in  large  masses.  Copper  crystallizes  in  the  iso- 
metric system,  but  the  crystals  are  usually  distorted  and  asso- 
ciated in  dendritic  groups.  The  forms  {100},  {111},  {110},  and 
{210}  can  sometimes  be  made  out. 

H.  =  2%.  Sp.  gr.  8.8 ±. 

Color.  Copper-red,  often  tarnished  and  also  encrusted  with 
alteration  products  such  as  cuprite  and  malachite.  Metallic 
luster.  Malleable. 

Chemical  Composition.     Copper,  often  with  a  little  silver. 

Blowpipe  Tests.     Fusible  (3)  to  a  malleable  globule. 

Soluble  in  dilute  HNO3  to  a  green  solution  with  the  evolution 
of  NO2.  With  an  excess  of  NH4OH,  the  nitric  acid  gives  a  deep 
blue  coloration  solution. 

Distinguishing  Features.  The  color  of  freshly  cut  copper  is 
distinctive. 

Uses.  Native  copper  is  an  important  source  of  copper  in  but 
one  locality  (Upper  Peninsula  of  Michigan),  where  immense 
quantities  of  copper  ore  are  produced  from  very  low  grade  ores. 


222        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Occurrence.  In  the  oxidized  zone  of  many  copper  mines 
formed  by  the  reduction  of  copper  compounds  in  solution,  which 
in  turn  were  formed  by  the  oxidation  of  chalcopyrite. 

2.  In  amygdaloidal  diabases  and  basalts  associated  with 
calcite,  datolite,  prehnite,  epidote,  zeolites,  and  sometimes  with 
silver.  The  Upper  Peninsula  of  Michigan  is  the  type  locality. 

Platinum,  Pt 

Form.  Platinum  is  found  in  rounded  grains,  scales,  and  irregu- 
lar lumps.  Cubic  crystals  have  been  found  but  are  exceedingly 
rare. 

H.  =  4K-        Sp.  gr.      15-19    (21,  if  pure). 

Color.  Light  steel  gray.  Luster  metallic.  Malleable.  Some 
varieties  are  magnetic  because  of  the  high  iron  content. 

Chemical  Composition.  Platinum,  often  alloyed  with  iron 
and  metals  of  the  platinum  group  (Ir,  Os,  Pd,  Rh). 


Pt 

Fe 

Pd 

Rh 

Ir 

Os 

Cu 

Urals  

80.9 

2.3 

1.6 

11.1 

tr 

1.0 

S  =  0.8 

Colombia  
California  

84.8 
79.8 

8.3 
9.4 

1.0 
0.3 

2.1 
3.4 

1.0 
4.3 

1.0 
1.1 

0.6 
0.3 

Blowpipe    Tests.     Infusible. 

Soluble  in  aqua  regia.  From  this  solution  KC1  precipitates 
K2PtCl6,  a  yellow  crystalline  powder  composed  of  minute 
octahedra. 

Distinguishing  Features.  Platinum  is  recognized  by  its 
high  specific  gravity,  and  its  refractory  nature. 

Uses.  Platinum  is  used  largely  for  chemical  apparatus  and  in 
jewelry,  but  also  for  some  industrial  purposes.  Native  platinum 
is  the  only  source  of  platinum.  The  Ural  Mountains  furnish 
practically  the  entire  supply. 

Occurrence.  1.  In  placer  deposits  along  with  gold,  magnetite, 
ilmenite,  zircon,  diamond,  etc.  Prominent  localities  are  the 


ELEMENTS 


223 


Ural  Mountains,    Colombia,   British  Columbia,  northern  Cali- 
fornia, and  southern  Oregon. 

2.  In  peridotites  or  dunites  with  chromite,  olivine,  and  ser- 
pentine. These  rocks  are  the  original  source  of  the  platinum 
of  placers.  Urals  and  British  Columbia. 

Iron,  Fe  to  (Fe,Ni) 

Form.  Iron  is  found  in  compact  or  spongiform  masses  and  in 
disseminated  grains.  Iron  crystallizes  in  the  isometric  system, 
but  distinct  crystals  are  exceedingly  rare.  Many  meteoric  irons 
have  an  octahedral  structure  which  is  revealed  by  etching  a 
polished  surface  with  nitric  acid,  and  is  due  to  varying  solubility 
of  several  different  alloys  of  iron  and  nickel. 

H.  =  4K-  Sp.gr.  7.5  ±. 

Color.  Steel-gray  to  iron-black,  often  covered  with  iron-rust. 
Luster,  metallic.  Malleable.  Attracted  by  the  magnet. 

Chemical  Composition.  Iron,  usually  alloyed  with  nickel. 
It  usually  contains  small  amounts  of  cobalt,  copper  carbon,  sul- 
fur, phosphorus,  etc. 


Fe 

Ni 

Co 

Cu 

s 

Mn 

C 

Silicates, 
Insol. 

Terrestrial  iron, 
Greenland 

91.7 

1.7 

0.5 

0.1 

0.1 

1.4 

3.9 

Meteoric  iron, 
Siberia  

88.4 

10.7 

0.5 

0.1 

tr. 

0.1 

0.1 

0.5 

Blowpipe  Tests.  Infusible.  Amber  borax  bead  in  O.F., 
bottle  green  in  R.F. 

Soluble  in  HC1  with  the  evolution  of  hydrogen.  Iron  becomes 
copper-coated  in  a  solution  of  copper  sulfate. 

Distinguishing  Features.  Iron  is  recognized  by  its  oxidized 
surface  and  bright  metallic  interior. 

Occurrence.  1.  In  meteorites,  either  as  the  main  constituent 
(iron  meteorites)  or  associated  with  silicates  such  as  olivine 
(iron-stone  meteorites) . 


224        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

2.  Terrestrial  iron  is  usually  formed  by  the  reduction  of  iron 
compounds.  The  only  prominent  locality  is  Disco  Island  off 
the  west  coast  of  Greenland,  where  large  masses  are  found  in  a 
basalt.  This  iron  constitutes  a  natural  steel  as  it  contains  the 
requisite  amount  of  carbon  and  shows  the  microscopic  structure 
characteristic  of  steel.  The  basalt  is  associated  with  coal  beds 
and  the  coal  has  doubtless  served  as  a  reducing  agent  of  the  iron 
in  the  basaltic  magma. 


2.  SULFIDS 

STIBNITE  Sb2S3 

Bismuthinite,  Bi2S3 

Molybdenite,  MoS2 

Argentite,  Ag2S 

GALENA,  PbS 

CHALCOC1TE,  Cu2S 

SPHALERITE,  ZnS 

Pentlandite  (Fe,Ni)S 

CINNABAR,  HgS 

Covellite,  CuS 

PYRRHOTITE,  FeS(S)x 

j  PYRITE,  FeS2 

I  Smaltite,  (Co,Ni)As2 

j  Marcasite  FeS2 

1  ARSENOPYRITE,  FeAsS 

Calaverite,  AuTe2 

The  sulfids  and  their  analogues,  the  selenids,  tellurids,  arse- 
nids,  etc.,  are  derivatives  of  H2S,  H2Se,  H2Te,  H2As,  etc.  They 
may  be  considered  as  salts  of  these  acids  or  as  sulfanhydrids  of 
sulfo-salts,  just  as  the  oxids  are  anhydrids  of  the  oxy-salts.  They 
are  mostly  sulfids  of  the  heavy  metals.  The  sulfids  may  be 
divided  into  two  classes:  (1)  the  sulfids  of  the  semi-metals,  and 
(2)  the  sulfids  of  the  metals.  Minerals  under  each  of  these  are 
arranged  according  to  increasing  number  of  sulfur  atoms  in  the 
formula.  Prominent  isomorphous  groups  are  indicated  by 
brackets. 

STIBNITE,  Sb2S3 

Form.     Stibnite  is  found  in  prismatic  or  acicular  crystals,  in 
columnar  or  bladed  aggregates,  and  in  granular  masses.     Crys- 
tals are  orthorhombic  (bipyramidal  class).     The  habit  is  long 
is  225 


226        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

prismatic  with  {110}  as  the  dominant  form  (110:lTO  =  89°  34'). 
Crystals  are  often  highly  modified  and  always  vertically  striated. 

Cleavage.  Perfect  in  one  direction  parallel  to  the  side  pina- 
coid  (010).  It  may  also  have  parting  parallel  to  (001)  which  is 
manifested  in  cross  lines  on  the  cleavage  surfaces. 

H.  =  2.  Sp.  gr.  4.5  ±. 

Color.  Lead  gray.  Luster,  brilliant  metallic  on  fresh  sur- 
face. Streak,  lead  gray. 

Chemical  Composition.  Antimony  sulfid,  Sb2S3;  (Sb  =  71.4 
per  cent.). 

Blowpipe  Tests.  On  charcoal  easily  fusible  (1),  gives  a  pale 
greenish-blue  flame,  dense  white  fumes,  and  a  white  sublimate 
close  to  the  assay.  In  the  open  tube  gives  S02  and  a  white  non- 
volatile sublimate  (Sb204)  on  the  under  side  of  the  tube.  In  the 
closed  tube  gives  a  dark  red  sublimate  of  antimony  oxysulfid 
(Sb2S20). 

Decomposed  by  HNOs  with  the  separation  of  a  white  precipi- 
tate of  metantimonic  acid  (HSbO3). 

Distinguishing  Features.  Stibnite  differs  from  similar  min- 
erals in  its  perfect  cleavage  in  one  direction,  and  its  easy  fusi- 
bility. (It  can  be  fused  in  the  flame  of  a  match).  It  is  often 
coated  with  pale  yellow  stibiconite,  a  product  of  oxidation. 

Uses.  Stibnite  is  the  principal  source  of  antimony  which  is 
used  extensively  in  the  manufacture  of  various  alloys.  Some 
of  these  alloys,  such  as  type-metal,  are  made  directly  from  anti- 
monial  lead  ores.  China  is  the  principal  producer  of  stibnite. 

Occurrence.  1.  As  a  vein  mineral  often  associated  with  pyrite, 
sphalerite,  galena,  cinnabar,  and  realgar  in  a  gangue  of  quartz, 
barite,  or  calcite.  Prominent  localities  for  specimens  of  stibnite 
are  Felsobanya,  Hungary,  and  Shikoku,  Japan.  At  the  latter 
locality  magnificent  crystals  over  a  foot  in  length  are  found. 

Bismuthinite,  Bi2S3 

Form.  Bismuthinite  is  isomorphous  with  stibnite,  and  greatly 
resembles  it. 


SULFIDS  227 

Cleavage.     In  one  direction  parallel  to  the  length. 

H.  =  2.  Sp.  gr.  6.4  ±. 

Color.     Lead  gray,  often  with  a  peculiar  yellowish  tarnish. 

Chemical  Composition.  Bismuth  sulfid,  Bi2S3;  (Bi  =  81.2 
per  cent.).  Some  varieties  contain  Se. 

Blowpipe  Tests.  Easily  fusible  (1),  gives  on  charcoal  a 
metallic  button  (malleable,  but  brittle  on  the  edges)  and  a  yellow 
coating.  Heated  with  iodid  flux  on  plaster  it  gives  a  purplish 
chocolate  sublimate  with  underlying  scarlet. 

Soluble  in  HNO3.  On  diluting  the  solution  with  water  a 
white  precipitate  is  formed. 

Distinguishing  Features.  Resembles  stibnite  and  is  only 
distinguished  from  it  by  blowpipe  or  chemical  tests. 

Uses.  Bismuthinite  is  probably  the  most  important  bis- 
muth mineral.  Bolivia  is  the  chief  producer. 

Occurrence.  1.  As  a  vein  mineral  associated  with  bismuth 
and  chalcopyrite  especially. 

Molybdenite,  MoS2 

Form.  Molybdenite  is  usually  found  in  foliated  masses  or  in 
disseminated  scales,  and  occasionally  in  hexagonal  crystals  of 
tabular  habit. 

Cleavage.     In  one  direction  parallel  to  {0001}. 

H.  =  IY2.  Sp.  gr.  4.7 ±. 

Color.  Bluish  lead  gray.  The  streak  on  glazed  porcelain  or 
glazed  paper  (the  thumb  nail  is  a  fair  substitute)  has  a  greenish 
tinge.  Cleavage  plates  are  flexible  and  sectile. 

Chemical  Composition.  Molybdenum  sulfid,  MoS2  (Mo  = 
60.0  per  cent.). 

Blowpipe  Tests.  Infusible.  On  charcoal  gives  a  white  subli- 
mate which  is  copper  red  near  the  assay.  Green  NaPOs  bead  in 
R.F.,  colorless  in  O.F. 

Decomposed  by  HNOa  with  the  formation  of  a  white  sublimate 
(MoO3)  which  is  soluble  in  NH4OH. 


228        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Distinguishing  Features.  Distinguished  from  graphite  by 
higher  specific  gravity  and  streak  on  glazed  paper. 

Uses.  Molybdenite  is  the  chief  source  of  molybdenum  which 
is  used  as  an  alloy  with  steel.  Australia  is  the  leading  producer 
of  molybdenite. 

Occurrence.  1.  In  pegmatites  and  surrounding  rocks,  espe- 
cially granite. 

2.  In  tin-stone  veins  with  cassiterite,  wolframite,  topaz,  etc. 

3.  In    contact-met  amor  phic    zones   between   limestones   and 
granites  associated  with  epidote,  chalcopyrite,  etc. 

Argentite,  Ag2S 

Form.  Occurs  massive,  incrusting,  more  rarely  in  rough  crys- 
tals. The  crystals  are  isometric,  the  only  common  form  being 
the  cube.  Cubes  are  often  arranged  in  parallel  position. 

H.  =  2%.  Sp.  gr.  7.3  ±. 

Color.  Dark  lead  gray,  dull  black  on  exposed  surface.  Luster, 
metallic.  Very  sectile. 

Chemical  Composition.  Silver  sulfid,  Ag2S;  (Ag  =  87.1  per 
cent.) 

Blowpipe  Tests.  Easily  fusible  (lj^).  On  charcoal  yields  a 
malleable  button  of  silver. 

Soluble  in  HN03  with  the  separation  of  S.  HC1  gives  a  white 
precipitate  (AgCl)  soluble  in  NH4OH. 

Distinguishing  Features.  Distinguished  by  its  perfect  sec- 
tility  and  metallic  luster. 

Uses.  Argentite,  the  silver  glance  of  the  miner,  is  an  impor- 
tant ore  of  silver  on  account  of  the  high  silver  content. 

Occurrence.  As  a  vein  mineral  associated  with  other  silver 
minerals,  and  pyrite,  galena,  sphalerite,  etc.  Freiberg,  Saxony. 

GALENA,  PbS 

Form.  Galena  occurs  in  well-formed  crystals  as  well  as  in 
cleavable  and  granular  masses.  Crystals  are  isometric  (hexocta- 
hedral  class) .  The  usual  forms  are  the  cube  { 100 } ,  the  octahedron 


SULFIDS 


229 


(111),  more  rarely  the  dodecahedron  {110},  and  the  trisocta- 
hedron  {221}.  The  habit  is  usually  cubic,  cubo-octahedral, 
or  octahedral,  as  shown  by  Figs.  380-384.  Small  octahedral 
crystals  are  sometimes  found  in  parallel  position  on  large  cubic 
crystals. 

Cleavage.     Perfect  cubic  cleavage. 

H.  =  2^.  Sp.  gr.  7.5  ±. 

Color.     Lead  gray,  often  tarnished.     Metallic  luster. 

Chemical  Composition.  Lead  sulfid,  PbS  (Pb  =  86.6  per  cent.) 
May  contain  zinc,  silver,  free  sulfur,  and  other  impurities.  If 
the  silver  is  chemically  combined,  it  is  present  in  amounts  less 
than  0.1  per  cent,  according  to  Guild. 

Blowpipe  Tests.  Easily  fusible  at  2,  giving  on  charcoal  a  malle- 
able button,  a  yellow  sublimate  (PbO)  near  the  assay,  and  a 
white  sublimate  (PbSCM  farther  from  the  assay. 


FIG.  380.  FIG.  381.  FIG.  382.  FIG.  383.  FIG.  384. 

FIGS.  380-384. — Galena  crystals. 

The  silver  may  be  obtained  by  cupellation  (see  p.  34). 

Decomposed  by  HC1  with  the  separation  of  PbCl2,  a  white 
crystalline  precipitate  soluble  in  hot  water.  Decomposed  by 
HNOa  with  the  separation  of  S  and  PbSO4. 

Distinguishing  Features.  It  is  an  easy  mineral  to  recognize 
on  account  of  its  cleavage  and  high  specific  gravity. 

Uses.  Galena  is  the  most  important  ore  of  lead;  argentifer- 
ous galena  is  one  of  the  most  important  silver  ores.  The  silver 
is  for  the-  most  part  present  as  included  silver  minerals.  South- 
eastern Missouri  and  the  Coeur  d'Alene  district  of  Idaho  are  the 
most  important  sources  of  galena. 


230        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Occurrence.  1.  In  veins  associated  with  sphalerite,  chalcopy- 
rite,  pyrite,  etc.,  often  with  barite  or  fluorite,  a  gangue  mineral. 
In  the  north  of  England  galena  occurs  with  fluorite,  barite,  cal- 
cite,  and  sphalerite  in  veins  in  Sub-carboniferous  limestone. 

2.  In  sedimentary  rocks  such  as  limestones,  shales,  and  sand- 
stones often  associated  with  sphalerite.     In  southeastern  Mis- 
souri galena  is  disseminated  through  Ordovician  limestone. 

3.  In  contact-metamorphic  zones  with  sphalerite. 

CHALCOCITE,  Cu2S 

Form.  Usually  fine-grained  compact  masses,  rarely  in  pseudo- 
hexagonal  orthorhombic  crystals,  which  are  sometimes  twinned. 

H.  =  2>i  Sp.gr.  =  5.78. 

Color.  Dark  lead-gray  with  black  tarnish.  Metallic  luster. 
Chalcocite  is  sub-sectile  (i.e.,  it  can  be  cut  with  a  knife,  but  not  so 
readily  as  argentite). 

Chemical  Composition.  Cuprous  sulfid,  Cu2S;  (Cu  =  79.8  per 
cent.).  It  may  contain  a  little  cupric  sulfid  in  solid  solution.  A 
little  iron  is  usually  present,  due  to  admixed  bornite,  chalcopyrite, 
or  pyrite. 

Blowpipe  Tests.  Fuses  at  2.  Unaltered  in  the  closed  tube. 
In  the  open  tube  gives  the  odor  of  SO2.  In  R.F.  on  charcoal 
gives  metallic  copper. 

Soluble  in  HNOs  giving  brown-red  fumes,  residue  of  S,  and  a 
green  solution. 

Distinguishing  Features.  A  compact  massive  mineral,  dis- 
tinguished by  its  imperfect  sectility,  especially  from  tetrahedrite, 
which  is  very  brittle. 

Uses.  Chalcocite  is  a  valuable  ore  of  copper  on  account  of  the 
high  percentage  of  copper.  At  the  Bonanza  Mine,  Kennecott, 
Alaska,  enormous  quantities  of  chalcocite  ore  running  70  per 
cent,  copper  are  being  mined.  It  is  a  prominent  mineral  in  the 
ores  at  Butte,  Montana,  and  also  occurs  in  the  disseminated 
"porphyry  copper"  ores  in  Arizona,  Utah,  and  Nevada. 


SULFIDS  231 

Occurrence.  1.  As  a  product  of  downward  secondary  enrich- 
ment, formed  at  the  expense  of  pyrite,  chalcopyrite,  or  bornite. 
Bingham,  Utah. 

2.  As  a  vein  mineral  associated  with  pyrite,  chalcopyrite, 
bornite,  and  covellite  and  is  often  formed  as  a  replacement  of 
these  minerals  by  ascending  solutions.  Butte,  Montana. 

On  heating  orthorhombic  chalcocite  (/3-Cu2S)  to  a  temperature 
of  91°C  or  above,  it  changes  to  an  isometric  form  («-Cu2S). 
Some  specimens  of  chalcocite  show  structures  on  polished  sur- 
faces which  prove  them  to  be  paramorphs  of  /3-Cu2S  after  «-Cu2S. 
This  makes  it  practically  certain  that  some  chalcocite  has  been 
formed  by  hydrothermal  ascending  solutions. 

SPHALERITE,  ZnS 

Form.  Sphalerite  crystallizes  in  the  hextetrahedral  class  of 
the  isometric  system.  Crystals  are  usually  distorted  and  diffi- 
cult to  decipher.  The  common  habits  are  tetrahedral  and  dodec- 
ahedral;  the  usual  forms  are  a{100),  djllO},  o{lll),  o 


FIG.  385.  FIG.  386. 

FIGS.  385,  386. — Sphalerite  crystals. 

and  raj  311}.  Figs.  385  and  386  represent  typical  crystals. 
Crystals  are  often  twinned  on  the  {111}  face  and  twinning  striations 
due  to  polysynthetic  twinning  on  this  face  are  sometimes 
observed. 

The  internal  structure  of  sphalerite  as  determined  by  JY-ray 
analysis  (Bragg  and  Bragg)  is  shown  in  Fig.  298,  p.  144. 

Cleavage.     Very  prominent  (dodecahedral  at  angles  of  60°). 

H.  =  3K  to  4.  Sp.  gr.  4.0  ±. 


232        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Color.  The  color  of  sphalerite  varies  from  white  to  black, 
depending  upon  the  amount  of  iron  present.  The  usual  color  is 
yellowish-brown  or  reddish-brown.  The  luster  varies  from 
adamantine  to  submetallic,  and  the  streak  from  pale  yellow  in 
yellow  varieties  to  dark  brown  in  the  black  varieties. 

Optical  Properties,  n  =  2.37.  Fragments  are  triangular, 
pale  yellow  to  brown,  and  isotropic  (dark  between  crossed  nicols). 
The  high  index  of  refraction  accounts  for  the  adamantine  luster. 

Chemical  Composition.  Zinc  sulfid,  ZnS;  (Zn  =  67.0  per 
cent.).  Usually  contains  iron  (up  to  as  high  as  20  per  cent.) 
which  replaces  zinc  isomorphously,  as  shown  in  the  formula 
(Zn,Fe)S.  Cadmium  is  another  common  impurity  present  in 
part  as  a  isomorphous  replacement  of  zinc  and  in  part  as  an 
associated  cadmium  sulfid  (xanthochroite)  occurring  as  an  amor- 
phous yellow  incrustation  on  sphalerite.  The  rare  elements  in- 
dium and  gallium  were  discovered  in  sphalerite.  The  following 
are  typical  analyses : 


Zn 

Fe 

S 

Misc. 

White:  Franklin,  New  Jersey  

67.5 

32.2 

Cd 

=  tr 

Yellow:  Schemnitz,  Hungary  

65.2 

0.5 

32.8 

Cd 

=  1.5 

Brown:  Roxbury,  Connecticut  

63.4 

3.6 

33.4 

Dark  brown:  Westphalia  

58.2 

8.2 

33.4 

Cu 

=  0.1;  Pb 

=  tr 

Black:  Felsobanya,  Hungary  

50.0 

15.4 

33.3 

Cd 

=  0.3;  Pb 

=  1.0 

Blowpipe  Tests.  Fusible  with  difficulty  (5) .  On  charcoal  gives 
a  white  sublimate,  which  is  yellow  when  hot.  This  sublimate 
heated  intensely  with  cobalt  nitrate  solution  gives  a  green  color 
(cobalt  zincate).  The  presence  of  cadmium  is  indicated  by  an 
iridescent  coating  on  charcoal. 

Soluble  in  HC1  with  the  evolution  of  H2S. 

Distinguishing  Features.  The  perfect  cleavage  together  with 
the  adamantine  luster  will  distinguish  sphalerite  from  all  other 
common  minerals.  It  somewhat  resembles  siderite  and  occasion- 
ally garnet. 


SULFIDS  233 

Uses.  Sphalerite  is  the  most  important  ore  of  zinc.  The 
Joplin  district  of  southwest  Missouri  is  the  principal  locality  in 
this  country. 

Occurrence.  1.  As  a  vein  mineral  associated  with  galena, 
chalcopyrite,  pyrite,  and  other  sulfids  in  a  gangue  of  quartz,  calcite, 
barite,  fluorite,  dolomite,  etc.  - 

2.  As  a  replacing  or  accessory  mineral  in  sedimentary  rocks, 
especially  limestones. 

3.  As    a    contact-metamorphic  mineral.     Magdalena  mines, 
New  Mexico. 

Pentlandite  (Fe,Ni)S 

Form.  A  massive  mineral  very  much  like  pyrrhotite  in 
appearance. 

Cleavage.     Octahedral. 

H.  =  3H  to  4.  Sp.  gr.  4.8  ±. 

Color.     Bronze-yellow  like  pyrrhotite.     Opaque,  non-magnetic. 

Chemical  Composition.  Iron  and  nickel  sulfid  (Fe,Ni)S. 
(Ni  =  20  to  40  per  cent.) 

Blowpipe  Tests.  Easily  fusible  (at  2)  to  a  magnetic  globule 
which  gives  the  bead  tests  for  iron  and  nickel. 

Soluble  in  nitric  acid  to  a  green  solution.  When  made  alka- 
line the  solution  gives  a  red  ppt.  with  dimethylglyoxime. 

Distinguishing  Features.  It  very  much  resembles  pyrrhotite 
but  is  distinguished  by  its  octahedral  cleavage  (or  parting)  and 
by  its  non-magnetic  character.  On  a  polished  surface  containing 
pyrrhotite  it  is  not  affected  by  HC1,  while  the  pyrrhotite  is. 

Uses.  Pentlandite  is  the  chief  ore  of  nickel.  It  is  extensively 
mined  in  the  Sudbury  district,  Ontario,  Canada  where  it  occurs 
intimately  associated  with  pyrrhotite  and  chalcopyrite.  This 
district  is  the  most  important  producer  of  nickel  ore;  its  only 
rival  is  New  Caledonia. 

Occurrence.  1.  As  a  magmatic  sulfid  associated  with,  and 
formed  later  than,  pyrrhotite.  Sudbury  District,  Ontario, 
Canada. 


234        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

CINNABAR,   HgS 

Form.  Cinnabar  usually  occurs  disseminated  through  the 
rock  and  in  massive  and  earthy  forms.  It  may  also  occur  in 
minute  crystals  in  cavities.  The  crystals  are  hexagonal  and  of 
variable  habit. 

Cleavage.  Perfect,  often  parallel  to  length  of  the  crystal. 
Even  in  massive  varieties  reflections  from  minute  cleavage  planes 
usually  may  be  seen  with  a  lens. 

H.  =  2^.  Sp.  gr.  8.0  ±. 

Color.  Scarlet  to  dark  red,  sometimes  black  when  impure. 
Luster,  adamantine  in  typical  specimens.  Streak,  vermilion. 

Optical  Properties.  nT(3.20)  -  na(2.85)  =  0.35.  Fragments 
are  red,  and  irregular  with  high  order  interference  colors.  The 
very  high  index  of  refraction  accounts  for  the  adamantine  luster. 

Chemical  Composition.  Mercuric  sulfid,  HgS;  (Hg  =  86.2  per 
cent.).  Clay  and  organic  matter  are  often  present  as  impurities. 

Blowpipe  Tests.  Volatile  if  pure.  In  the  closed  tube  with 
dry  sodium  carbonate,  cinnabar  gives  a  sublimate  of  metallic 
mercury  (little  globules  when  rubbed  with  a  wire). 

Soluble  in  HNO3. 

Distinguishing  Features.  Cinnabar  is  distinguished  from 
other  red  colored  minerals  by  its  high  specific  gravity,  adamantine 
luster,  and  cleavage. 

Uses.  Practically  the  only  ore  of  mercury.  The  principal 
producing  localities  are  Almaden,  in  Spain,  Idria,  in  Austria, 
New  Almaden  and  New  Idria  in  California. 

Occurrence.  1.  In  deposits  formed  near  the  surface.  The 
more  common  associated  minerals  are  pyrite,  marcasite,  stibnite, 
and  mercury.  The  gangue  minerals  are  chalcedony,  opal,  barite, 
and  calcite. 

Covellite,  CuS 

Form.     Tabular   hexagonal    crystals   are   rare;   the   mineral 
usually  occurs  as  a  dissemination  or  incrustation. 
H.  =  IY2  to  2.  Sp.  gr.  =  4.68. 


SULFIDS  235 

Color.  Deep  indigo  blue,  sometimes  almost  black.  Luster, 
metallic  pearly  to  earthy. 

Chemical  Composition.  Cupric  sulfid,  CuS;  (Cu  =  66.4  per 
cent.). 

Blowpipe  Tests.  Fusible  at  2^-  In  the  closed  tube  gives  a 
sublimate  of  sulfur  (distinction  from  chalcocite).  On  charcoal 
burns  with  a  blue  flame,  gives  off  S(>2,  and  leaves  a  residue  of 
metallic  copper. 

Soluble  in  HNO3  to  a  green  solution. 

Distinguishing  Features.  The  dark  blue  color  is  distinctive 
for  a  metallic  mineral. 

Uses.  One  of  the  minor  ores  of  copper.  It  is  prominent  in 
some  of  the  Butte  mines. 

Occurrence.  1.  A  characteristic  mineral  of  the  zone  of 
downward  secondary  sulfid  enrichment.  In  occurs  associated 
with  chalcopyrite,  bornite,  and  chalcocite. 

2.  In  veins  with  other  copper  minerals  and  probably  formed  by 
ascending  solutions.  Butte,  Montana. 

PYRRHOTITE,  FeS(S)x 

Form.  Pyrrhotite  is  usually  massive  and  without  distinct 
cleavage,  though  it  may  have  a  platy  structure.  Pseudohexa- 
gonal  orthorhombic  crystals  of  tabular  habit  are  known,  but  are 
very  rare. 

H.  =  4.  Sp.  gr.  4.6  ±. 

Color.  Bronze-yellow.  Luster,  metallic.  Attracted  by  the 
magnet  but  usually  only  slightly  so. 

Chemical  Composition.  Ferrous  sulfid,  usually  with  a  little 
excess  of  sulfur  present  in  solid  solution  and  expressed  by  the 
formula  FeS(S)*.  (Fe  =  60-63.6  per  cent.).  It  frequently 
contains  nickel  which  is  present  as  admixed  pentlandite. 

Blowpipe  Tests.  Fuses  at  3  to  a  magnetic  globule,  gives 
fumes  of  SO2.  In  the  closed  tube  gives  little  or  no  sulfur  (dis- 
tinction from  pyrite). 

Soluble  in  HNO3. 


236        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Distinguishing  Features.  The  bronze  color  and  slightly 
magnetic  character  of  pyrrhotite  are  distinctive.  It  can  be  dis- 
tinguished from  pyrite  by  the  fact  that  it  is  scratched  by  the 
knife. 

Uses.  The  pyrrhotite  itself  has  no  special  uses,  but  the  inti- 
mately associated  pentlandite  which  occurs  with  it  at  Sudbury, 
Canada,  is  the  world's  chief  source  of  nickel. 

Occurrence.  1.  In  plutonic  basic  igneous  rocks,  such  as 
gabbros  and  norites,  as  a  late  magmatic  mineral  formed  by 
the  replacement  of  the  silicate  minerals. 

2.  In  high-temperature  veins  and  replacement  deposits. 

3.  In  contact-metamorphic  deposits. 

4.  In  meteorites.     This  variety,    FeS,  without  an  excess  of 
sulfur,  is  known  as  troilite. 

PYRITE  GROUP— ISOMETRIC 

Pyrite,  FeS2;  smaltite,  CoAs2;  chloanthite,  NiAs2;  cobaltite, 
CoAsS;  and  gersdorffite,  NiAsS,  constitute  an  isomorphous  group 
as  they  crystallize  in  the  diploid  class  of  the  isometric  system  in 
cubes  and  pyritohedrons,  and  have  a  hardness  of  5^  to  6^  and 
a  specific  gravity  of  5  to  6.5.  There  are  also  intermediate  com- 
pounds such  as  (Co,Fe)As2,  (Ni,Fe)As2,  and  (Co,Fe)AsS.  The 
general  formula,  then,  can  be  written  (Fe,Co,Ni)  (As,Sb,S)2. 

PYRITE,  FeS2 

Form.  Pyrite  is  often  well  crystallized  and  furnishes  the 
typical  example  of  the  diploid  class  of  the  isometric  system. 
The  most  common  forms  are  the  cube  a  {100},  pyritohedron 
e{210},  octahedron  0(111} ,  diploid  s{321),  diploid  {421},  and 
trapezohedron  {211}.  The  habit  is  nearly  always  either 
cubic,  pyritohedral,  or  octahedral.  Figures  387  to  394  represent 
typical  crystals.  The  cube  faces  are  commonly  striated  as  in 
Fig.  407.  The  {hkO}  forms  are  so  characteristic  of  pyrite  that 
they  are  called  pyritohedrons.  Of  these,  the  pyritohedron  {210J 
is  the  most  common.  Penetration  twins  of  the  pyritohedron 


SULFIDS 


237 


with  the  a-axis  as  the  twinning  axis  are  occasionally  found.     (See 

Fig.  257,  page  127.) 

H.  =  6  to  6^.  Sp.  gr.  5.0  ±  (5.027  if  pure) 

Color.     Brass  yellow.    Luster,  metallic.     Streak,  greenish  to 

brownish-black. 
Chemical  Composition.     Iron  disulfid,  FeS2;  (Fe  =  46.6  per 

cent.).     May  contain  copper,  cobalt,  nickel,  arsenic,  and  gold; 

but  of  these  only  the  cobalt  and  nickel  are  chemically  combined. 


lllllll 


FIG.  387. 


FIG.  388. 


FIG.  389. 


FIG.  390. 


FIG.  391.  FIG.  392.  FIG.  393. 

FIGS.  387-394. — Pyrite  crystals. 


FIG.  394. 


In  the  so-called  cupriferous  pyrite,  the  copper  is  in  the  form  of 
chalcopyrite.     The  following  are  typical  analyses: 


Fe 

S 

Cu 

Misc. 

French  Creek,  Penn  

44.2 

54.1 

0.05 

Arnsberg,  Germany  

46.4 

51.4 

1.0 

Mn  =  0.5;  Co  =  0.1;  As  =  0.6 

Cornwall,  Penn  

44.5 

53.4 

2.4 

Blowpipe  Tests.     Fusible  at  3  to  a  magnetic  globule.     On  char- 
coal  it  burns   with   a  blue   flame  and  gives  off   SO2.     In  the 


238        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

closed  tube,  pyrite  gives  a  sublimate  of  sulfur  and  leaves  a  mag- 
netic residue. 

Soluble  in  cold  HNO3,  but  no  sulfur  separates  unless  the  acid  is 
heated. 

Distinguishing  Features.  Pyrite  is  distinguished  from  pyr- 
rhotite  and  chalcopyrite  by  superior  hardness,  and  from  marca- 
site,  its  dimorph,  by  crystal  form  and  solubility  in  cold  HN03. 

Uses.  Pyrite  is  used  extensively  in  the  manufacture  of  sul- 
furic  acid,  which  is  the  basis  of  many  chemical  industries.  Spain, 
Norway,  Portugal,  United  States,  and  Italy  are  the  principal 
producers.  Pyrite  is  also  an  important  low-grade  copper  ore  at 
many  localities.  The  copper  is  present  as  disseminated  chalco- 
pyrite. Pyrite  is  also  often  gold-bearing. 

Occurrence.  1.  As  a  vein  mineral  associated  with  other 
sulfids. 

2.  As  a  secondary  mineral  in  igneous  rocks,  especially  in  the 
country  rock  around  ore  deposits. 

3.  As  a  dissemination  in  sedimentary  rocks,  such  as  shales 
and  limestones,  often  replacing  organisms. 

4.  As  bedded  deposits  in  metamorphic  rocks,  perhaps  formed 
from  original  pyrrhotite. 

5.  As  a  contact-metamorphic  mineral  often  associated  with 
hematite  and  magnetite. 

Smaltite,  (Co,Ni)As2 

Form.  Smaltite  is  usually  massive  without  any  cleavage,  but 
occasionally  is  found  in  cubic  crystals. 

H.  =  5^.  Sp.gr.  6.2  ±. 

Color.     Tin  white  to  steel  gray.    Luster  metallic. 

Chemical  Composition.  Cobalt  and  nickel  arsenid,  (Co,Ni)- 
As2,  varying  from  CoAs2  (Co  =  28.1  per  cent.)  to  NiAs2  (Ni  = 
28.1  per  cent.).  If  the  latter  predominates,  the  mineral  is  called 
chloanthite.  Iron  and  sulfur  are  usually  present  in  small  amounts. 
The  following  analyses  illustrate  the  range  in  composition. 


SULFIDS 


239 


Co 

Ni 

Fe 

As 

S 

Misc. 

Atacama,  Chili  

24.1 

1.2 

4.1 

70.8 

0.1 

Cu  =  0.4 

10  1 

8  5 

5  1 

69  7 

4  7 

Cu  =  0.9;  Bi  =  1.0 

Schneeberg,  Saxony  

4.2 

24.9 

0.7 

68.4 

1.1 

Bi  =  0.2 

Blowpipe  Tests.  On  charcoal  gives  off  arsin  and  fuses  at 
to  a  magnetic  globule,  which  colors  the  borax  bead  blue.  In 
the  closed  tube  it  gives  an  arsenic  mirror  if  strongly  heated.  In 
the  open  tube  it  gives  minute  octahedral  crystals  of  As2O3. 

Soluble  in  HNO3  to  a  rose-red  solution. 

Distinguishing  Features.  Smaltite  resembles  arsenopyrite 
and  can  only  safely  be  distinguished  by  blowpipe  or  chemical 
tests. 

Uses.  Smaltite  is  the  chief  ore  of  cobalt  which  is  used 
principally  as  a  blue  pigment  (a  cobalt  silicate  called  smalt.) 
Cobalt,  Ontario,  is  the  principal  producer. 

Occurrence.  1.  As  a  vein  mineral  usually  with  silver  or 
bismuth  and  in  a  gangue  of  calcite.  Smaltite  is  often  coated 
with  erythrite,  a  hydrous  cobalt  arsenate  known  as  "cobalt 
bloom." 

MARCASITE  GROUP—  ORTHORHOMBIC 

The  following  minerals:  marcasite,  FeS2;  arsenopyrite,  FeAsS; 
lollingite,  FeAs2,*  glaucodot  (Co,Fe)AsS;  safflorite,  CoAs2;  and 
rammelsbergite,  NiAs2,  constitute  an  isomorphous  group  parallel 
to  the  pyrite  group.  They  are  orthorhombic  in  crystallization, 
have  a  hardness  of  5  to  6^,  and  are  tin-white  to  brass-yellow  in 
color.  Only  the  first  two  of  these  minerals  are  considered,  as 
the  others  are  rare. 

Marcasite, 


Form.  Marcasite  occurs  in  orthorhombic  crystals,  in  crystal- 
line aggregates,  and  in  rounded  concretionary  masses.  Crystals 
are  usually  tabular  in  habit  and  often  elongated  in  the  direction 


240        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

of  the  a-axis.  Figures  395  to  397  represent  typical  crystals  with 
the  forms:  c{001),  m{110),  t>{013}.  ram(110:lTO)  =  74°  55'. 
Twins  with  mjllO}  as  twin-plane  are  common. 

H.  =  6to6^.  Sp.  gr.  4.9 ±. 

Color.  Pale  brass  yellow  with  a  greenish  tinge.  Almost  tin- 
white  when  cleaned  with  dilute  HC1  (distinction  from  pyrite 
which  is  yellow).  Luster  metallic. 

Chemical  Composition.  Iron  disulfid,  FeS2;  (Fe  =  46.6  per 
cent.).  Analyses  often  show  small  amounts  of  arsenic. 

Blowpipe  Tests.  The  same  as  for  pyrite,  except  that  it  is  de- 
composed by  cold  nitric  acid  with  the  separation  of  sulfur. 


m 


FIG.  395.  FIG.  396.  Fio.  397 

FIGS.  395-397. — Marcasite  crystals. 

Distinguishing  Features.  Marcasite  is  distinguished  from 
pyrite  by  crystal  form  and  by  difference  in  color  and  behavior 
with  nitric  acid. 

Uses.  If  found  in  sufficient  quantity,  marcasite  could  be 
used  in  the  manufacture  of  sulfuric  acid. 

Occurrence.  1.  In  sedimentary  rocks  or  associated  with  coal 
beds,  often  in  concretions.  Dover,  England. 

2.  As  a  vein  mineral  formed  near  the  surface  at  a  low  tem- 
perature and  probably  from  acid  solutions.  Joplin  district, 
Missouri. 

ARSENOPYRITE,  FeAsS 

Form.  Arsenopyrite  is  found  in  well-formed  crystals,  as  well 
as  in  disseminated  grains  and  compact  masses.  The  crystals 


SULFIDS 


241 


are  orthorhombic,  similar  in  habit  and  angles  to  marcasite. 
Figures  398  and  399  represent  typical  crystals  with  the  forms 
m(110l,  c{001},  and  u{QU}. 

H.  =  5K  to  6.  Sp.gr.  6.0  ±. 

Color.    Tin-white  to  light  steel-gray.     Luster,  metallic. 

Chemical  Composition.  Iron  arsenid-sulfid,  FeAsS;  (Fe  = 
34.3,  As  =  46.0,  S  =  19.7).  It  often  contains  cobalt  and  grades 
into  glaucodot. 

Blowpipe  Tests.  On  charcoal  fuses  (at  2)  to  a  magnetic  glob- 
ule and  gives  off  arsin.  In  the  closed  tube  on  gentle  heating 
gives  a  red  sublimate  (AsS),  but  on  further  heating  an  arsenic 
mirror  is  formed.  In  the  open  tube  minute  crystals  of  As203 
are  deposited  and  SO2  also  formed. 


m 


FIG.  398.  FIG.  399. 

FIGS.  398,  399. — Arsenopyrite  crystals. 

Soluble  in  HNOs  with  the  separation  of  S. 

Distinguishing  Features.  Crystals  of  arsenopyrite  resemble 
marcasite  but  are  distinguished  by  color  of  fresh  surface.  Mas- 
sive arsenopyrite  resembles  smaltite  and  often  can  only  be  dis- 
tinguished by  blowpipe  tests. 

Uses.  Arsenopyrite  is  the  chief  source  of  the  white  arsenic 
(As2Os)  of  commerce.  At  Deloro,  Canada,  arsenopyrite  is  a 
gold  ore. 

Occurrence.  1.  As  an  intermediate-temperature  vein  mineral 
associated  with  pyrite,  chalcopyrite,  galena,  and  other  sulfids. 
Mother  Lode  of  California. 

2.  In  high-temperature  veins. 

3.  In  granite-pegmatites. 

16 


242        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Calaverite,  AuTe2 

Form.  Calaverite  usually  occurs  in  small,  striated,  elongated 
crystals  along  seams.  The  crystals  are  complex  monoclinic; 
some  of  the  faces  have  such  very  high  indices  that  the  law  of 
rational  indices  has  been  questioned. 

Fracture,  subconchoidal.     No  cleavage. 

H.  -  2H-  Sp.  gr.  9.0  ±. 

Color.  Pale  brass  yellow,  somewhat  resembling  pyrite. 
Metallic  luster. 

Chemical  Composition.  Gold  tellurid,  AuTe2j  (Au  =  44  per 
cent.).  It  always  contains  some  silver,  usually  from  2  to  4  per 
cent. 

Blowpipe  Tests.  Easily  fusible  (at  1)  on  charcoal  to  a  yellow 
button  of  gold,  giving  dense  white  fumes  and  coloring  the  flame 
bluish-green.  The  powdered  mineral  dropped  into  hot  con- 
centrated H2S04  gives  a  purplish-red  coloration. 

Soluble  in  aqua  regia  with  the  separation  of  a  little  AgCl. 

Distinguishing  Features.  Calaverite  is  distinguished  from 
pyrite  by  its  inferior  hardness  and  elongated  crystals. 

Uses.  An  important  ore  of  gold.  At  Cripple  Creek,  Colorado 
it  is  the  chief  source  of  gold.  It  also  occurs  in  West  Australia 
associated  with  sylvanite  ( Au AgTe4) .  The  name  is  derived  from 
Calaveras  county,  California,  where  it  was  first  found  in  the 
Stanislaus  Mine. 

Occurrence.  1.  As  a  vein  mineral.  At  Cripple  Creek, 
fluorite  is  a  common  associate. 


3.  SULFO-SALTS 

CHALCOPYRITE,   CuFeS2 
BORNITE,  Cu6FeS4 

Jamesonite  Pb4FeSbeSu 

Pyrargyrite,  Ag3SbS3 

TETRAHEDRITE,  Cu3SbS3  +  s(Fe,  Zn)6Sb2S9 
Stephanite,  Ag5SbS4 

Polybasite,  (Ag,Cu)i6Sb2Sii 

Enargite,  Cu3AsS4 

Under  the  sulfo-salts  are  included  certain  compounds  of  sulfur, 
salts  of  hypothetical  acids  which  may  be  derived  from  ordinary 
oxygen  acids  by  replacing  S  for  O,  as  these  two  elements  are 
similar  chemically. 

Three  classes  of  these  compounds  may  be  distinguished:  (1) 
Sulfoferrites,  derivatives  of  H3FeS3  analogous  to  ferrous  acid, 
H3FeO3;  (2)  Sulfarsenites  and  sulfantimonites,  derivatives  of 
H3AsS3  and  H3SbS3  analogous  to  arsenious  acid,  H3AsO3  and 
antimonous  acid,  H3SbO3;  (3)  Sulfarsenates  and  sulfantimon- 
ates,  derivatives  of  H3AsS4  and  H3SbS4,  analogous  to  arsenic 
acid,  H3AsO4,  and  antimonic  acid,  H3SbO4. 

There  are  also  condensed  acids  derived  from  the  above 
mentioned  acids  by  the  subtraction  or  addition  of  H2S,  just 
as  condensed  acids  may  be  derived  from  oxygen  acids  by  the 
subtraction  or  addition  of  H20.  Thus  chalcopyrite,  CuFeS2, 
is  a  salt  of  HFeS2  derived  from  H3FeS3  (H3FeS3  -  H2S  =  HFeS2). 
Jamesonite  is  a  salt  of  H5Sb3S7  (3H3SbS3  -  2H2S).  Stephanite 
is  a  salt  of  H5SbS4  (H3SbS3+H2S).  Polybasite  is  a  salt  of 
H16Sb2Sn  (2H3SbS3+5H2S). 

The  sulfo-salts  are  sometimes  considered  as  double  sulfids. 
Thus  pyrargyrite  Ag3SbS3  is  written  3Ag2S-Sb2S3. 

About  sixty  sulfo-salt  minerals  are  known,  but  most  of  them 
are  rare. 

243 


244        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


CHALCOPYRITE,  CuFeS2 

Form.  Chalcopyrite  occurs  in  crystals,  in  masses,  and  dis- 
seminated through  the  rock.  The  crystals  belong  to  the  tetra- 
gonal system,  scalenohedral  class,  but  are  pseudotetrahedral  and 
pseudo-octahedral  in  form.  Fig.  400  represents  a  common  type  of 
crystal  with  the  forms  pjlll)  and  z{201j.  This  is  a  tetragonal 
bisphenoid  and  is  distinguished  from  a  tetrahedron  by  the 
striations. 


H. 


to  4. 


Sp.  gr.  4.2  ±. 


FIG.     400  . — Chal- 
copyrite crystal. 


Color.  Brass  yellow,  often  with  an  iridescent  tarnish,  hence 
the  name  "  peacock  copper."  Metallic  luster. 

Chemical  Composition.  Cuprous  sulfoferrite,  CuFeS2;  (Cu  = 
34.5  per  cent.).  Variations  from  this  formula 
are  usually  due  to  admixed  pyrite. 

Blowpipe  Tests  On  charcoal  fusible  (at  2) 
to  a  magnetic  globule  which  heated  with 
sodium  carbonate  gives  a  copper  button.  In 
the  closed  tube  it  decrepitates  and  gives  a 
sublimate  of  sulfur. 

Soluble  in  HNOs  to  a  green  solution  with 
the  separation  of  sulfur  from  which  NH.4OH 
gives  a  red-brown  precipitate  and  blue  solution. 

Distinguishing  Features.  Chalcopyrite  is  distinguished  from 
pyrite  by  difference  in  color  and  inferior  hardness,  and  from  gold 
by  its  brittleness. 

Uses.  Chalcopyrite  is  one  of  the  principal  ores  of  copper  and 
the  most  widely  distributed  copper  mineral. 

Occurrence.  1.  As  a  vein  mineral  associated  with  pyrite, 
galena,  sphalerite,  tetrahedrite,  bornite,  etc. 

2.  In  basic  igneous  rocks  with  pyrrhotite,  as  a  late  mag- 
matic  mineral.  Sudbury,  Canada. 

4.  In  fahlbands  of  schists  and  gneisses. 

5.  As  a  contact  mineral  with  magnetite  and  hematite. 


SULFO-SALTS  245 

BORNITE,  Cu5FeS4 

Form.  Bornite  occurs  in  masses  and  disseminated  specks, 
very  rarely  in  rough  cubic  crystals. 

H.  =  3.  Sp.  gr.  5.1±. 

Color.     A  red-brownish  bronze  with  purple  tarnish. 
Metallic  luster.     Slightly  sectile. 

Chemical  Composition.  Copper  sulfoferrite,  Cu5FeS4;  (Cu  = 
63.3  per  cent.).  Analyses  vary  widely  due  to  intermixture  with 
chalcopyrite  and  chalcocite. 

Blowpipe  Tests.  Fusible  (at  2J^)  on  charcoal  R.F.  to  a  mag- 
netic globule.  In  the  closed  tube  gives  a  faint  sublimate  of  sulfur. 

Soluble  in  HNO3  to  a  green  solution  with  the  separation  of  S 
from  which  NH4OH  gives  a  red-brown  precipitate  and  a  blue 
solution. 

Distinguishing  Features.  The  peculiar  color  of  bornite  and  its 
purple  tarnish  distinguishes  it  from  all  other  minerals. 

Uses.  Bornite  is  an  important  ore  of  copper.  It  is  usually 
intimately  associated  with  chalcopyrite  or  chalcocite.  Butte, 
Montana. 

Occurrence.  1.  As  a  vein  mineral  associated  with  chalcocite, 
chalcopyrite,  and  pyrite.  Butte,  Montana. 

2.  As  a  late  magmatic  mineral  associated  with  chalcopyrite. 
Engels  mine,  Plumas  county,  California. 

3.  As  a  contact-metamorphic  mineral  between  limestones  and 
igneous  rocks. 

Jamesonite,  Pb4FeSb6Si4 

Form.  Jamesonite  occurs  in  delicate  capillary  crystals  and  in 
columnar  #nd  compact  masses. 

Cleavage.  If  cleavage  is  distinct,  it  is  transverse  to  the  length 
of  the  crystals. 

H.  =  2M.  Sp.gr.  5.7+ . 

Color,    Lead-gray.     Metallic  luster. 


246        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Chemical  Composition.  Lead  and  iron  sulfantimonite,  Pb4- 
FeSb6SJ4  (Pb  =  40.3  per  cent.). 

Blowpipe  Tests.  On  charcoal  easily  fusible  (at  1)  giving  white 
and  yellow  coatings.  In  the  closed  tube  gives  a  yellow  subli- 
mate of  sulfur  and  a  dark-red  sublimate  of  Sb2S2O.  With 
sodium  carbonate  on  charcoal  it  gives  a  lead  button. 

Soluble  in  HC1  with  the  evolution  of  H2S.  On  cooling  the 
solution  needle  crystals  of  PbCl2  separate.  Decomposed  by 
HNOs  with  the  separation  of  a  white  residue  (HSbO3). 

Distinguishing  Features.  Jamesonite  resembles  stibnite,  but 
has  no  distinct  cleavage  parallel  to  the  length  of  the  crystals. 

Occurrence.     1.  As  a  vein  mineral.     Sevier  county,  Arkansas. 


Pyrargyrite,  Ag3SbS3 

Form.  Pyrargyrite  often  occurs  in  small  well-defined  crystals 
belonging  to  the  ditrigonal  pyramidal  class.  The  habit  is  usually 
prismatic,  and  the  hexagonal  prism,  { 1120},  the 
dominant  form.  Opposite  ends  of  the  crystal 
are  differently  terminated.  Figure  401  repre- 
sents a  typical  crystal;  the  striations  indicate 
the  hemimorphic  character. 
H.  =  2J£.  Sp.  gr.  5.8  ±. 

Color.  Dark  red  to  black.  Translucent  red 
on  thin  edges.  Streak  purple-red.  Luster 
metallic-adamantine. 

Optical    Properties.     n7(3.08)  -  na(2.88)  = 
0.20.     Fragments  are  irregular  and  red  in  color, 
with  high  order  interference  colors. 

Chemical  Composition.  Silver  sulfantimonite,  Ag3  SbSa ;  (Ag  = 
59.9  per  cent.).  Arsenic  replaces  antimony  to  some  extent. 
The  corresponding  sulfarsenite  is  called  proustite.  Together 
they  constitute  the  ruby  silver  group. 

Blowpipe  Tests.  On  charcoal  fuses  easily  (at  1)  to  a  globule  of 
silver  sulfid  giving  a  white  sublimate.  This  globule  with  sodium 


SULFO-SALTS 


247 


carbonate  in  R.F.  gives  a  silver  button.  Heated  intensely  in  the 
closed  tube,  it  gives  a  slight  red  sublimate. 

Decomposed  by  HNO3  with  the  separation  of  sulfur  and  a  white 
residue. 

Distinguishing  Features.  Pyrargyrite  is  distinguished  from 
other  silver  minerals  (except  .proustite,  Ag3AsS3)  by  the  red 
fragments  and  streak,  and  from  cuprite  and  cinnabar  by  blow- 
pipe tests. 

Uses.  A  valuable  ore  of  silver,  often  associated  with  argentite, 
stephanite,  and  polybasite. 

Occurrence.  1.  As  a  vein  mineral  often  formed  at  a  late 
stage  by  ascending  solutions.  Tonopah,  Nevada. 

TETRAHEDRITE,  Cu3SbS3  +z(Fe,Zn)6Sb2S9 

Form.  Tetrahedrite  occurs  in  masses,  and  also  in  crystals 
belonging  to  the  hextetrahedral  class  of  the  isometric  system. 


FIG.  402.  FIG.  403.  FIG.  404.  FIG.  405. 

FIGS.  402^405. — Tetrahedrite  crystals. 

The  common  forms  are  the  tetrahedron  oflll},  the  tristetrahe- 
dron  w{211},  and  the  dodecahedron  djllOj.  Figures  402 
to  405  represent  typical  crystals. 

H.  =  3^.  Sp.  gr.  4.7±. 

Color.     Dark  iron-gray.     Metallic  luster.     Brittle. 

Chemical  Composition.  Copper  sulfantimonite,  Cu3SbS3-|-z 
(Fe,Zn)6Sb2S9,  where  x  =  Ko  to  ^.  The  copper  is  often 
replaced  by  silver  and  the  antimony  by  arsenic.  It  grades 
into  tennantite,  the  corresponding  sulfarsenite.  The  following 
analyses  illustrate  the  wide  variation  in  chemical  composition. 


248        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Cu 

Sb 

S 

As 

Fe 

Zn 

Ag 

Misc. 

Fresney  d'Oisans  

45.4 

28  8 

24  5 

tr 

1  3 

Pb  =  0  1 

38  0 

23  9 

25  8 

2  9 

0  8 

7  3 

0  6 

Machetillo,  Chili  
Cabarrus  Co.,  N.  C  
Poracs,  Hungary  

36.7 
30.7 
32.8 

20.7 
17.8 
30.2 

25.3 
25.5 

24.9 

6.5 
11.6 

1.2 
1.4 
5.9 

6.9 
2.5 

2.9 
10.5 
0.1 

Hg  =  5.6 

Blowpipe  Tests.  Easily  fusible  (at  lj^)  giving  dense  white 
fumes  and  a  white  sublimate  near  the  assay.  The  residue  heated 
with  sodium  carbonate  in  R.F.  gives  metallic  copper.  In  the 
closed  tube  it  gives  a  dark  red  sublimate  (Sb2S2O). 

Soluble  in  HN03  to  a  green  solution  with  the  separation 
of  sulfur  and  a  white  residue,  HSbO3. 

Distinguishing  Features.  Tetrahedrite  is  apt  to  be  confused 
with  chalcocite.  It  is  very  brittle,  while  chalcocite  is  somewhat 
sectile. 

Uses.  Tetrahedrite  is  an  ore  of  copper  and  silver  known  to 
miners  as  "gray  copper."  Highly  argentiferous  tetrahedrite  is 
called  freibergite. 

Occurrence.  1.  As  a  vein  mineral  often  associated  with  chal- 
copyrite,  galena,  sphalerite,  and  siderite.  Cornwall,  England, 
and  Oruro,  Bolivia,  are  prominent  localities. 

2.  In  fahlbands  of  schists.  Fahlore  is  a  synonym  of  tetrahe- 
drite. 

Stephanite,  Ag5SbS4 

Form.  Stephanite  occurs  disseminated,  compact  massive, 
more  rarely  in  crystals.  The  crystals  are  orthorhombic,  but 
pseudohexagonal  and  short  prismatic  in  habit. 

H.  =  2%.  Sp.gr.  6.2±. 

Color.     Dark  gray  to  black.     Very  brittle.     Metallic  luster. 

Chemical  Composition.  Silver  sulfantimonite,  Ag5SbS4,'  (Ag 
=  68.5  per  cent.). 


SULFO-SALTS  249 

Blowpipe  Tests.  On  charcoal  easily  fusible  (at  1)  to  a  globule 
giving  dense  fumes  and  a  white  sublimate  of  Sb2Os.  The  globule 
heated  with  sodium  carbonate  R.F.  gives  a  silver  button. 

Decomposed  by  HN03  with  the  separation  of  sulfur  and  a  white 
residue. 

Distinguishing  Features.  Stephanite  resembles  argentite  and 
tetrahedrite.  Argentite  is  sectile,  and  tetrahedrite  harder  than 
stephanite. 

Uses.  A  valuable  ore  of  silver.  It  was  a  prominent  mineral 
in  the  Comstock  Lode  of  Nevada.  Stephanite  is  known  to 
miners  as  " brittle  silver." 

Occurrence.  1.  As  a  vein  mineral  formed  at  a  late  stage 
by  ascending  solutions. 

Polybasite,  (Ag,Cu)16Sb2Sn 

Form.  Polybasite  usually  occurs  in  monoclinic  pseudo-hexag- 
onal crystals  of  tabular  habit  with  triangular  striations  on  the 
basal  pinacoid. 

H.  =  2H.  Sp.gr.  6.1  + . 

Color.     Iron  black.     Metallic  luster. 

Optical  Properties.  n>1.93.  Very  thin  fragments  are  deep 
red  translucent. 

Chemical  Composition.  Silver  and  copper  sulfantimonite 
(Ag,Cu)i6Sb2Sn;  (Ag  =  about  70  per  cent.).  Polybasite  usually 
contains  arsenic  and  thus  grades  into  pearceite,  (Ag,Cu)j6As2Sn. 

Blowpipe  Tests.  On  charcoal  fuses  easily  (at  1)  to  a  globule 
and  gives  a  white  sublimate.  The  globule  heated  with  sodium 
carbonate  gives  a  metallic  button.  In  order  to  get  pure  silver 
it  is  necessary  to  cupel  the  button. 

Decomposed  by  HNO3  with  the  separation  of  sulfur  and  a  white 
residue  (HSbO3). 

Distinguishing  Features.  The  tabular  crystals  with  triangular 
markings  are  distinctive. 

Uses.  Polybasite  is  an  ore  of  silver.  It  occurs  at  Tonopah, 
Nevada,  and  at  several  mines  in  Colorado. 


250        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Occurrence.  1.  As  a  vein  mineral,  formed  at  a  late  stage  by 
ascending  solutions.  Aspen,  Colorado. 

Enargite,  Cu3AsS4 

Form.  Enargite  occurs  in  columnar  masses  and  occasionally 
in  prismatic  orthorhombic  crystals. 

Cleavage.  Prominent  (in  two  directions  at  angles  of  82°  to 
each  other). 

H.  =  3.  Sp.  gr.  4.4±. 

Color.     Dark  gray  to  black. 

Chemical  Composition.  Copper  sulfarsenate,  CiisAsSij  (Cu  = 
48.3  per  cent.).  It  usually  contains  a  little  antimony  and  a  little 
iron.  The  corresponding  sulfantimonate  is  a  rare  mineral  known 
as  famatinite. 

Blowpipe  Tests.  Easily  fusible  at  1 ;  sulfur  dioxid  masks  the 
odor  of  arsin.  In  the  closed  tube  gives  a  sublimate  of  sulfur.  In 
the  open  tube  it  deposits  minute  octahedral  crystals  of  As2O3 
with  adamantine  luster  and  862  gas  is  formed. 

Soluble  in  HNO3. 

Distinguishing  Features.  The  columnar  structure  and  good 
cleavage  are  distinctive.  It  is  distinguished  from  stibnite  by  its 
darker  color. 

Uses.  Enargite  is  an  ore  of  copper,  occurring  at  Butte, 
Montana,  and  at  many  localities  in  South  America.  Near  Butte 
white  arsenic  (As203)  is  recovered  from  smelter  smoke. 

Occurrence.  1.  In  veins  and  replacement  deposits  formed  at 
intermediate  depths  and  temperatures.  At  Tintic,  Utah, 
enargite  is  the  original  source  of  several  copper  arsenate  minerals. 


4.  HALOIDS 

The  haloids  comprise  chlorids,  bromids,  iodids,  and  fluorids 
which  are  salts  of  HC1,  HBr,  HI,  and  HF  respectively.  Com- 
paratively few  haloids  occur  in  nature,  but  several  of  them  are 
very  common  minerals.  All  the  minerals  are  normal  anhydrous 
salts  with  the  exception  of  carnallite. 

A.  Normal  anhydrous  haloids 

HALITE,  NaCl 
Sylvite,  KC1 

Cerargyrite,  AgCl 
FLUORITE,  CaF2 
Cryolite,  Na3AlF6 

B.  Basic  and  hydrous  haloids 

Carnallite,       KMgCl3  6H2O 

HALITE,  NaCl 

Form.  Halite  occurs  in  crystals,  and  in  cleavable,  granular, 
and  fibrous  masses.  Crystals  are  isometric,  usually  cubes  (Fig. 
406),  sometimes  hopper-shaped  (Fig.  407),  rarely  in  octahedrons 
or  cubo-octahedrons  (Fig.  408). 

Cleavage.  The  perfect  cubic  cleavage  is  a  marked  feature  of 
halite.  A  dodecahedral  {110}  parting  is  developed  by  pressure 
applied  on  the  cube-edges  by  a  hammer  or  in  a  vise  (Fig.  265, 
page  131). 

H.  =  2M-  Sp.  gr.  2.1±. 

Color.  Colorless  and  white,  often  reddish  or  gray,  and  some- 
times deep  blue  in  patches. 

Optical  Properties.  Isotropic.  n  =  1.54.  Recrystallizes  from 
a  water  solution  in  squares,  often  hopper-shaped  (Fig.  490),  which 
are  dark  between  crossed  nicols  and  have  low  relief  in  clove 
oil. 

251 


252        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Chemical  Composition.  Sodium  chlorid,  NaCl;  (Na  =  39.4 
per  cent.)-  Halite  may  contain  MgCl2,MgSO4,CaCl2,  and  Ca- 
SO4.  It  is  to  these  impurities  that  the  deliquescence  of  table  salt 
is  due. 

Blowpipe  Tests.  Fuses  easily  (at  1),  giving  an  intense  yellow 
flame.  With  CuO  in  NaPO3  bead  it  gives  an  azure-blue  flame. 

Soluble  in  cold  water. 

Distinguishing  Features.  Distinguished  from  most  minerals 
by  its  cubic  cleavage.  It  has  a  pleasant  saline  taste  and  not  the 
sharp  taste  of  sylvite. 

Uses.  Halite  is  the  chief  source  of  sodium  compounds  used 
extensively  in  the  manufacture  of  soap  and  glass,  and  also  as 


FIG.  406. 


FIG.  407.  FIG.  408. 

FIGS.  406-409. — Halite  crystals. 


FIG.  409. 


table  salt  and  as  a  preservative.  Salt  brines  furnish  bromin. 
Salt  is  obtained  (1)  directly  by  the  mining  of  rock-salt  as  at 
Petite  Anse,  Louisiana,  and  Lyons,  Kansas;  (2)  by  pumping 
brine  to  surface  and  evaporating  as  at  Syracuse,  New  York,  and 
Hutchinson,  Kansas;  and  (3)  by  solar  evaporation  as  at  Great 
Salt  Lake,  Utah. 

Occurrence.  1.  Occurs  in  beds  associated  with  anhydrite, 
gypsum,  and  occasionally  with  other  chlorids  and  sulfates. 
These  deposits  are  formed  by  the  evaporation  of  sea- water. 
Important  localities  are  Stassfurt,  Germany;  Wieliczka,  Poland; 
Cheshire,  England;  western  New  York;  Saginaw,  Michigan; 
and  central  Kansas. 


HALOIDS 


253 


Sylvite,  KC1 

Form.  Sylvite  occurs  in  cleavable  and  granular  masses  and  in 
well-formed  cubic  or  cubo-octahedral  (like  Fig.  408)  crystals. 
Etch-figures  indicate  that.sylvite  belongs  to  the  gyroidal  class  of 
the  isometric  system. 

Cleavage.     Perfect  cubic  cleavage. 

Color.  Colorless  or  white,  sometimes  with  bluish  opalescence. 
Taste,  sharp  saline. 

H.  =  2.  Sp.  gr.  2.0±. 

Optical  Properties.     Isotropic.     n  =  1.49     (clove    oil).     Re- 
crystallizes  from  water  solution  in  square  crystals  with  a  tendency 
toward  skeleton  crystals  (Fig.  410).     These 
are  dark  between  crossed  nicols  and  have 
moderate  relief  in  clove  oil. 

Chemical  Composition.  Potassium 
chlorid,  KC1;  (K  =  52.4  per  cent.).  It 
may  contain  NaCl. 

Blowpipe  Tests.  Fuses  easily  (at  1J^), 
coloring  the  flame  violet.  With  CuO  in  a 
NaP03  bead  it  gives  an  azure-blue  flame. 

Soluble  in  cold  water. 

Distinguishing  Features.  It  is  re- 
cognized by  its  cubic  cleavage  and  its  sharp  saline  taste. 

Uses.     Used  as  a  fertilizer  and  a  source  of  potassium  salts. 

Occurrence.  1.  In  salt  beds  with  halite,  anhydrite,  kainite, 
and  carnallite.  It  is  sometimes  a  secondary  mineral  formed 
from  carnallite  (KMgCl3-6H2O).  Stassfurt,  Germany,  and 
Mulhouse,  Alsace. 

2.  As  a  volcanic  sublimate  on  lava.     Vesuvius. 


FIG.    410.  —  Sylvite 
recrystallized. 


Cerargyrite,  AgCl 

Form.     Cerargyrite  usually  occurs  as  a  thin  crust  or  seam,  but 
small  cubic  crystals  are  also  found. 
H.  =  2.  Sp.  gr.  5.5±. 


254        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Coloiv  Gray,  greenish,  or  violet.  Luster,  waxy  to  adaman- 
tine. Very  sectile. 

Optical  Properties.  Isotropic.  n  =  2.06.  May  be  ham- 
mered to  a  thin  sheet  which  is  translucent,  but  dark  between 
crossed  nicols.  A  few  drops  of  NH4OH  will  give  minute  octahe- 
dral crystals  (AgCl). 

Chemical  Composition.  Silver  chlorid,  AgCl  (Ag  =  75.3  per 
cent.). 

Blowpipe  Tests.  On  charcoal  fuses  easily  (at  1),  giving  a  silver 
button.  A  fragment  touched  with  a  NaPO3  bead  saturated  with 
CuO  gives  an  intense  azure-blue  flame. 

Insoluble  in  acids,  but  soluble  in  NH4OH. 

Distinguishing  Features.  Cerargyrite  is  easily  overlooked 
but  on  close  inspection  its  appearance  and  sectile  character  are 
distinctive. 

Uses.  An  ore  of  silver  in  the  western  United  States,  Mexico, 
and  Chili.  The  miner's  name  for  it  is  "horn  silver." 

Occurrence.  1.  A  mineral  characteristic  of  the  upper  part  of 
ore  deposits.  It  is  formed  by  the  action  of  chlorid-bearing 
meteoric  waters  on  other  silver  minerals,  and  therefore  is  promi- 
nent in  arid  regions.  Poorman  mine,  Idaho. 

FLUORITE,  CaF2 

Form.  Fluorite  usually  occurs  in  cleavable  masses,  but  also 
often  in  distinct  crystals.  The  crystallization  is  isometric 
(hexoctahedral  class).  Usual  forms:  a{  100},  /{310J,  £{421}, 
djllO),  o{lll}.  The  habit  is  practically  always  cubic;  the 
other  forms  are  subordinate.  Octahedral  crystals  are  rare. 
At  some  localities,  apparent  octahedra  are  built  up  of  minute 
cubes  in  parallel  position.  Figures  411  to  414  represent  typical 
crystals  of  fluorite.  Figure  414  is  a  penetration  twin  with  the 
cube  diagonal  as  twin  axis.  Vicinal  faces  with  high  indices  such 
as  {32-1-0}  are  often  found  on  these  crystals. 

Cleavage.  One  of  the  most  important  characters  of  fluorite  is 
the  perfect  octahedral  cleavage.  On  a  cube  this  will  show  as 


HALOIDS 


255 


triangular  faces  at  the  vertices,  or  at  least  as  cracks  in  this 
direction,  as  shown  in  Fig.  411. 

H.  =  4.  Sp.  gr.  3.2 ±    (3.18,  if  pure) 

Color.  Usually  colorless  or  some  tint  or  shade  of  violet  or 
green,  rarely  yellow,  brown,  blue  or  pink.  The  color  is  prob- 
ably due  to  hydrocarbons.  Some  crystals  from  Cumberland  are 
green  by  transmitted  light,  but  blue  by  reflected  light.  This 
property,  also  possessed  by  some  aniline  colors  such  as  red-ink, 
is  known  as  fluorescence,  a  name  derived  from  the  mineral 
fluorite.  Some  varieties  of  fluorite  are  also  phosphorescent,  that 
is,  after  being  heated,  continue  to  emit  light  in  the  dark. 


FIG.  411. 


FIG.  412. 


FIG.  413. 


FIG.  414. 


Optical  Properties.  Iso tropic,  n  =  1.434,  hence  high  relief 
in  clove  oil.  Fragments  are  triangular,  colorless,  and  dark  be- 
tween crossed  nicols.  (See  Fig.  371,  page  205.) 

Chemical  Composition.  Calcium  fluorid  CaF2;  (F  =  48.9  per 
cent.).  Impurities  are  usually  calcite,  dolomite,  barite  or  quartz. 
Free  fluorin  has  been  detected  in  some  fluorite.  Fluorite  is  the 
only  common  fluorid  occurring  in  nature. 

Blowpipe  Tests.  In  the  closed  tube  decrepitates.  Fuses  (at 
3)  to  an  enamel  coloring  the  flame  red. 

Distinguishing  Features.  Fluorite  is  distinguished  by  the 
cubic  crystals,  octahedral  cleavage,  and  specific  gravity,  which  is 
a  little  higher  than  the  average  non-metallic  mineral. 

Soluble  in  H2SO4  with  evolution  of  HF,  which  etches  glass. 
Dilute  H2SO4  added  to  a  hydrochloric  acid  solution  of  the  mineral 
gives  a  crystalline  precipitate  of  hydrous  calcium  sulfate. 

Uses.     The  main  use  of  fluorite  is  a  flux  in  iron  smelting  and 


256        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

foundry  work.  Western  Kentucky  and  southern  Illinois  are  the 
principal  sources  of  fluorite  in  this  country.  Minor  uses  are  the 
manufacture  of  enamels,  opalescent  glass,  and  hydrofluoric  acid. 
Moissan  in  his  work  on  fluorin  used  vessels  made  of  fluorite. 

Occurrence.  1.  As  a  vein  mineral  associated  with  galena, 
sphalerite,  calcite,  and  barite.  Typical  localities  are  fissure  veins 
in  the  limestone  of  western  Kentucky  and  lead  mines  in  the  north 
of  England,  where  magnificent  museum  specimens  are  found. 

2.  In  tin-stone  veins  associated  with  cassiterite,  apatite,  topaz, 
and  lepidolite.     Zinnwald,  Bohemia. 

3.  In  limestones.     St.  Louis,  Missouri. 

Cryolite,  3NaF-AlF3 

Form.  Massive  and  in  pseudo-cubic  (monoclinic)  crystals 
often  in  parallel  position. 

Cleavage.     Imperfect  in  three  directions  at  nearly  right  angles. 

H.  =  2K-  Sp.  gr.  3.0  ±. 

Color.     White,  sometimes  brown.     Translucent. 

Optical  Properties.  n$  =  1.36.  Low  relief  in  water  (for 
water,  n  =  1.333).  Double  refraction  very  weak.  Fragments 
are  roughly  rectangular  or  irregular.  Interference  colors  are  first 
order  gray. 

Chemical  Composition.  Sodium  aluminum  fluorid,  3NaF-  A1F3- 
(Al  =  12.8  per  cent.,  Na  =  32.8). 

Blowpipe  Tests.  Easily  fusible  (  at  1)  giving  an  intense  yellow 
flame. 

Soluble  in  H2S04  with  the  evolution  of  HF. 

Distinguishing  Features.  The  translucent  white  masses 
resemble  fluorite  but  lack  the  good  cleavage  of  the  latter.  Frag- 
ments placed  in  water  look  as  though  they  had  partially  dissolved. 

Uses.  Formerly  used  as  a  source  of  aluminum,  but  now  used 
as  a  bath  in  the  electrolytic  production  of  aluminum  from  bauxite. 
It  is  also  used  in  the  manufacture  of  sodium  and  aluminum  salts 
at  Natrona,  Pa.  The  mineral  is  shipped  from  Greenland. 

Occurrence*     1.  In  granite  pegmatites.     The  most  important 


HALOIDS 


257 


locality  is  Ivigtut,  in  southern  Greenland,  where  an  immense 
vein-like  mass  of  cryolite  containing  siderite,  sphalerite,  galena, 
etc.,  occurs  in  a  porphyritic  granite.  It  also  occurs  at  St. 
Peter's  Dome  in  El  Paso  county,  Colorado. 

Carnallite,  KMgCl3.6H2O 

Form.  Massive  or  granular.  Crystals  (orthorhombic)  are 
very  rare. 

Cleavage.     No  cleavage,  but  has  conchoidal  fracture. 

H.  =  1.  Sp.gr.  1.6  ±. 

Color.  Colorless  or  reddish.  Luster,  greasy.  Very  deli- 
quescent. 

Optical  Properties.  717(1. 49)  -  na(1.46) 
=  0.03.  Recrystallized  from  water  solu- 
tion, it  forms  in  order  (1)  isotropic  squares 
of  KC1,  (2)  rectangular  twinned  crystals  of 
KMgCl3-6H2O,  and  (3)  streaked  aggregates 
of  MgCl2  (Fig.  415). 

Chemical  Composition.  Hydrous  potas- 
sium magnesium  chlorid  KMgCl3-6H2O  or 
KCl-MgCl2-6H2O;  (KC1  =  26.8  per  cent.) 
(H2O  =  39.0  per  cent.). 

Blowpipe  Tests.  Fusible  at  1^,  coloring  the  flame  violet. 
With  CuO  in  a  NaPO3  bead  it  gives  an  azure-blue  flame.  Gives 
abundant  water  in  the  closed  tube. 

Soluble  in  water. 

Distinguishing  Features.  Carnallite  is  distinguished  by  its 
bitter  taste  and  lack  of  cleavage. 

Uses.  Carnallite  is  used  as  a  fertilizer  and  in  the  manufacture 
of  potassium  salts.  KC1  crystallizes  out  of  a  water  solution  of 
carnallite. 

Occurrence.  1.  In  salt  beds  associated  with  anhydrite, 
halite,  sylvite,  and  kainite.  Stassfurt,  Prussia,  is  the  most 
prominent  locality. 


FIG.  415. 


17 


5.  OXIDS 

QUARTZ,  Si02 

CHALCEDONY,  SiO2 

OPAL,  Si( 

Tridymite  SiO2 

Cristobalite  SiO2 

Ice,  H2O 

Cuprite,  Cu2O 

f  CORUNDUM,  A1208 

\  HEMATITE,  Fe2O3 

Turyite,  Fe2O3(H2O)» 

f  CASSITERITE,  SnO2 

\  RutUe,  TiO2 

Pyrolusite,  MnO2 

Stibiconite,  Sb2O4(H2O)x 

Among  the  oxids  are  some  of  the  most  common  and  widely 
distributed  minerals.  The  silica  minerals  are  placed  first,  and 
after  them  the  monoxids,  R20  and  RO,  the  sesquioxids,  R2O3,  and 
the  dioxids,  RO2  in  the  order  named. 

The  minerals  of  the  spinel  group,  sometimes  considered  as 
double  oxids  of  the  type  ROR203,  are  placed  in  a  separate 
division,  the  aluminates,  etc. 

QUARTZ,  SiO2 

Form.  Crystals  of  quartz  are  very  common,  both  large  and 
small,  loose  and  attached.  There  are  crystalline  aggregates  of 
various  kinds  as  well  as  massive,  granular,  and  compact  varieties. 

Quartz  crystallizes  in  the  trigonal  trapezohedral  class  of  the 
hexagonal  system.  6  =  J..099.  Usual  forms:  rjlOll},  z{OlTl[, 
ra{10lO},  s{1121}±  z[5161}.  Interfacial  angles:  rar(10TO:10ll) 
=  38°_  13';jr(10ll:Tl01)  =  85°  46';  rz(10ll:  Oil!)  =  46°  16'; 
ms(WlO:  1121)  =  37°  58_';  raz(1010  :  0111)  =  66°  52';  mx(WW 
5161)  =  12°  1';  mm(10lO  :  OlIO)  =  60°  0'.  Figures  416-421 

258 


OXIDS 


259 


represent  typical  crystals.  The  habit  varies  from  prismatic 
to  pyramidal.  The  two  rhombohedrons  r  and  z  are  often  in  equal 
combination  (Figs.  416  ,  419)  and  apparently  form  a  hexagonal 
bipyramid.  The  s  face  at  alternate  vertices  proves  the  trigonal 
character.  Figures  420  and  421  are  more  complex  with  x}  { 6151 J 
and.si{2lll}. 

Cleavage,  practically  absent  (an  imperfect  cleavage  parallel  to 
r  is  occasionally  noticed). 

H.  =  7.  Sp.gr.  2.66 ±. 

Color,  more  often  white  or  colorless,  but  may  be  any  color. 
Luster,  vitreous.  Transparent  to  translucent,  rarely  opaque. 


FIG.  416. 


FIG.  418. 


FIG.  418. 


FIG.  419. 


Optical  Properties.  n7(1.553)  -  na(  1.544)  =  0.009.  Double 
refraction  rather  weak.  Fragments  are  irregular,  with  low 
relief  in  clove  oil  (n  >  clove  oil)  and  upper  first-order  interference 
colors. 

In  thick  basal  ( J_  to  c-axis)  sections  quartz  shows  rotary  polari- 
zation, i^.}  in  monochromatic  light  a  section  only  becomes  dark 
by  rotating  one  nicol.  For  red  light  the  angle  of  rotation  is 
13°  for  each  millimeter  of  thickness.  .  Sections  from  crystals 
like  Fig.  421  rotate  the  plane  to  the  right,  and  those  from  crystals 
like  Fig.  420  rotate  the  plane  to  the  left. 

Chemical  Composition.     Silica  or  silicon  dioxid,  SiO2.     Varia- 


260        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


tions  in  analyses  are  due  to  inclusions  such  as  chlorite,  tourma- 
line, rutile,  etc. 

Blowpipe  Tests.  Infusible  even  on  the  thinnest  edges.  When 
fused  with  an  equal  volume  of  sodium  carbonate,  effervesces 
and  gives  a  colorless  glass  (Na2C03  +  SiO2  =  CO2  +  Na2- 
SiO3.)  Insoluble  in  a  NaPO3  bead. 

Insoluble  in  ordinary  acids.     Soluble  in  HF. 


FIG.  420. — Left-handed  quartz 
crystal. l 


FIG.  421. — Right-handed  quartz 
crystal. 


Distinguishing  Features.  Quartz  often  resembles  calcite  but 
is  distinguished  by  its  superior  hardness  and  lack  of  cleavage. 
From  the  other  forms  of  silica  it  is  distinguished  by  its  indices 
of  refraction. 

Uses.  Quartz  in  the  form  of  rock  crystal,  amethyst,  and 
smoky  quartz  is  used  for  ornamental  purposes,  in  the  form  of  rock 
crystal  for  optical  apparatus,  in  the  form  of  sand  for  glass-making, 
and  in  the  form  of  pulverized  quartz  for  pottery  and  porcelain 
and  as  an  abrasive. 

1  This  figure  is  drawn  as  if  the  axes  had  been  rotated  to  the  right  instead  of  to  the  left,  as 
ordinarily.    This  brings  out  the  enantimorphous  relation  to  the  right-handed  crystal. 


OX  IDS  261 

Occurrence.  1.  As  normal  constituent  of  the  acid  igneous 
rocks  (rhyolites  and  granites).  (/3-quartz.) 

2.  As  an  abnormal  constituent  of  the  basic  igneous  rocks,  espe- 
cially basalts  (quartz  basalts). 

3.  As  a  vein  mineral,  often  the  gangue  of  ores.     Quartz  is 
the  most  common  vein  mineral.,    (a-quartz). 

4.  As  the  chief  constituent  of  sandstones  and  quartzites. 

5.  As  a  replacement  mineral  in  various  rocks,  often  occurring  as 
pseudomorphs  after  various  minerals,  and  as  petrifactions. 

6.  As  the  chief  constituent  of  river  and  beach  sands. 

CHALCEDONY,  SiO2 

Form.  Chalcedony  occurs  in  compact  masses  and  in  cavities 
in  colloform  crusts.  Although  chalcedony  is  never  found  in 
distinct  crystals,  it  is  crystalline  as  the  examination  of  thin  sec- 
tions or  fragments  in  polarized  light  will  show. 

Fracture,  more  or  less  conchoidal.     No  cleavage. 

H.  =  7.  Sp.  gr.  2.6  ±. 

Color,  colorless,  white,  or  any  color,  often  banded  and  varie- 
gated. Red  and  brown  varieties  are  called  jasper,  and  the 
banded  and  variegated  varieties,  agate.  Translucent  to  opaque. 
Luster,  waxy  to  dull. 

Optical  Properties.  n7(1.543)  -  na(1.532)  =  0.011.  Double 
refraction  rather  low.  Fragments  are  irregular  with  n  slightly 
lower  than  clove  oil.  The  aggregate  structure  with  low  order 
interference  colors  in  spots  and  streaks  is  highly  characteristic 
of  chalcedony  and  usually  distinguishes  it  from  quartz. 

Chemical  Composition.     Silica,  SiO2. 

Blowpipe  Tests.  Same  as  for  quartz  except  that  it  gives  a 
small  amount  of  water  in  the  closed  tube. 

Distinguishing  Features.  Chalcedony  is  distinguished  from 
most  minerals  of  similar  appearance  by  its  greater  hardness. 
From  the  other  silica  minerals  it  is  distinguished  by  its  dull 
luster.  (Quartz  has  vitreous  luster  and  opal,  greasy  luster.) 


262        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Uses.  Agate,  chrysoprase  (apple-green,  translucent  chalce- 
dony), and  jasper  are  used  as  ornamental  stones. 

Occurrence.  1.  As  a  secondary  mineral  in  seams  and  cavities 
of  various  rocks,  especially  the  volcanic  igneous  rocks. 

2.  As  chert  or  flint  and  jasper  occurring  in  concretions,  lenses 
or  layers  in  sedimentary  rocks.     The  origin  is  doubtful.     In 
the  Joplin  district  the  zinc  ores  occur  in  a  brecciated  chert,  which 
covers  large  areas. 

3.  As  a  low-temperature  vein  mineral,  often  the  gangue  of  gold, 
silver,  and  mercury  ores. 

OPAL,  SiO2(H2O)x 

Form.  Opal  usually  occurs  in  seams  and  cavities,  but  is  also 
disseminated  and  massive.  It  is  one  of  the  typical  amorphous 
minerals  and  so  in  cavities  it  often  has  a  colloform  structure. 

Fracture,  conchoidal.     No  cleavage. 

H.  =  5K  to  6M-  Sp.  gr.  2.1+  (very  light). 

Color,  white,  colorless,  or  almost  any  color.  Usually  translu- 
cent. Luster,  more  or  less  greasy. 

Optical  Properties.  Isotropic.  n  =  1.45.  Fragments  are 
irregular,  usually  dark  between  crossed  nicols,  and  have  high 
relief  in  clove  oil  (n  less  than  clove  oil,  Becke  test).  Some  varie- 
ties, notably  hyalite,  show  weak  double  refraction,  which  is  due 
to  strain.  Opal  is  often  intimately  mixed  with  chalcedony. 

Chemical  Composition.  Hydrous  silica  SiO2- (H^O^  with 
water  varying  from  3  to  12  per  cent.  Like  most  amorphous 
minerals  it  is  very  apt  to  contain  impurities.  The  following 
are  typical  analyses: 


SiOi 

H20 

A1203 

Fe2Oa 

CaO 

MgO 

Waltsch   Bohemia 

95  5 

3  0 

0.8 

0.2 

Washington  Co  ,  Ga 

91  9 

5.8 

1.4 

0.9 

Faroe  Islands               

88.7 

8.0 

1.0 

0.5 

1.5 

Meronitz,  Bohemia  

83.7 

11.5 

3.6 

1.6 

0.7 

OXIDS  263 

Blowpipe  Tests.  Infusible,  but  becomes  opaque.  In  the 
closed  tube  yields  water. 

Insoluble  in  the  ordinary  acids.  Soluble  in  HF  and  also  soluble 
in  KOH. 

Distinguishing  Features.  Opal  may  resemble  chalcedony 
but  has  a  lower  specific  gravity,-  is  a  little  softer,  and  usually  has 
a  greasy  luster. 

Uses.  The  opal  with  play  of  colors  known  as  precious  opal 
and  also  the  red  or  fire-opal  are  well  known  gems.  The  best 
precious  opals  are  found  in  New  South  Wales  and  in  Hungary, 
while  the  fire-opal  is  found  principally  in  Mexico. 

Occurrence.  1.  As  a  characteristic  mineral  in  cavities  and 
along  the  seams  of  igneous  rocks. 

2.  In  volcanic  tuffs  as  a  replacement  of  wood  (opalized  wood 
or  wood  opal). 

3.  As  siliceous  sinter  (geyserite)  formed  around   hot  springs 
and  geysers.     Yellowstone  National  Park  is  a  prominent  locality. 

4.  As  the   principal   constituent   of    diatomaceous  earth    or 
diatomite.     Diatoms  and  radiolaria  secrete  casts  of  opal  silica. 

Tridymite,  SiO2 

Form.  Tridymite  usually  occurs  in  the  form  of  minute 
crystals.  The  habit  is  pseudohexagonal  tabular;  twinned  crys- 
tals are  common.  The  high-temperature  /3-tridymite  is  hexa- 
gonal but  on  cooling  to  the  low-temperature  a-tridymite 
(probably  orthrombic)  it  retains  the  hexagonal  form. 

H.  =  7.  Sp.  gr.  =  2.27. 

Color,  colorless. 

Optical  Properties.  nT(1.473)  -  n«(1.469)  =  0.004.  Frag- 
ments are  six-sided  plates  or  irregular  with  fair  relief  in  clove  oil. 
The  double  refraction  is  very  weak. 

Chemical  Composition.  Silica,  SiO2,  the  same  as  that  of 
quartz. 

Blowpipe  Tests.     Infusible  before  the  blowpipe. 

Insoluble  in  ordinary  acids. 


264        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Distinguishing  Features.  Optical  tests  are  necessary  to  dis- 
tinguish tridymite  from  quartz  and  cristobalite.  The  indices  of 
refraction  of  tridymite  are  less  than  1.480,  while  those  of  cristo- 
balite are  greater  than  1.480. 

Occurrence.  1.  In  volcanic  igneous  rocks  usually  in  cavities 
and  probably  produced  by  hot  gases  after  the  main  period  of 
rock  formation.  Obsidian  Cliff,  Yellowstone  National  Park. 

Cristobalite,  SiO2 

Form.  Cristobalite  occurs  in  spherical  aggregates  or  in 
minute  pseudo-octahedral  crystals.  The  high-temperature  /?- 
cristobalite  is  isometric,  but  on  changing  to  a-cristobalite  it 
retains  the  isometric  form.  It  is  sometimes  found  in  pseudo- 
morphs  after  tridymite. 

H.  =  7.  Sp.  gr.  =  2.33. 

Color.     White  subtranslucent. 

Optical  Properties.  ny (1. 487)  -n«(l. 484)  =  0.003.  Fragments 
are  irregular  with  fair  relief  in  clove  oil.  The  double  refraction 
is  very  weak. 

Chemical  Composition.  Silica,  Si02,  the  same  as  that  of 
tridymite  and  quartz. 

Blowpipe  Tests.  Infusible  before  the  blowpipe.  On  heating 
it  becomes  somewhat  transparent  and  on  cooling  it  suddenly 
becomes  subtranslucent  again.  This  change  is  due  to  the  change 
of  the  high-temperature  /3-cristobalite  to  a-cristobalite.  Insoluble 
in  ordinary  acids. 

Distinguishing  Features.  The  characteristic  behavior  before 
the  blowpipe  distinguishes  it  from  similar  minerals.  In  occurrence 
and  general  characters  it  is  much  like  tridymite,  but  its  indices  of 
refraction  are  greater  than  1 .480  while  those  of  tridymite  are  less 
than  1.480. 

Occurrence.  1.  In  volcanic  igneous  rocks  usually  in  cav- 
ities and  probably  formed  by  hot  gases  after  the  main  period 
of  rock  formation.  Tehama  County,  California. 


OXIDS 


265 


Stability  Relations  of  the  Silica  Minerals 

Silica  exists  in  at  least  seven  well-defined  polymorphous  forms, 
each  of  which  is  stable  between  certain  temperature  limits  as 
shown  in  Fig.  422.  Ordinary  vein  quartz  is  stable  up  to  575°C. 


I!// 

8}  i  J/ 
o '  /  / 


58  Variable       °£  g  J| 

Temperature          3  S 

FIG.  422. — Stability  diagram  of  the  silica  minerals.     (After  Fenner.) 

At  that  temperature  there  is  a  sudden  change  in  the  indices  of 
refraction  and  some  of  the  other  properties,  and  it  passes  into 
the  form  known  as  j8-quartz.  This  form  is  the  one  char- 


266        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

acteristic  of  igneous  rocks,  especially  rhyolite  porphyry  and 
granite  porphyry.  No  specimens  of  0-quartz  exist  at  ordinary 
temperature  for  it  has  changed  to  a-quartz.  At  870°C. 
0-quartz  changes  suddenly  to  /32-tridymite,  and  at  1470°C.  this 
changes  to  0-cristobalite.  At  1625°C.  cristobalite  melts  to 
silica  glass.  On  cooling,  /3-cristobalite  changes  at  tempera- 
tures ranging  from  220°  to  275°C.  to  another  form  called 
a-cristobalite. 

Similarly  /32-tridymite  changes  to  /Si-tridymite  at  163°C. 
and  this  again  to  a-tridymite  at  117°C.  The  cristobalite  and 
tridymite  in  mineral  collections  are  each  the  a-forms  but 
usually  retain  the  crystal  habit  of  the  /3-forms.  The  rela- 
tion of  chalcedony  to  the  other  forms  of  silica  is  uncertain  and 
opal  can  not  be  treated  at  all  from  the  standpoint  of  the  phase 
rule.  It  is  a  two-phase  system,  for  it  consists  of  solution  of  water 
in  amorphous  silica  and  is  of  colloidal  origin.  The  diagram  of 
Fig.  422  has  been  worked  out  in  the  Geophysical  Laboratory  of 
the  Carnegie  Institution  of  Washington. 

Ice,  H2O 

Form.  Ice,  the  solid  form  of  H20,  occurs  in  frost  and  snow 
crystals  and  in  massive  and  granular  forms.  Ice  (and  snow) 
crystallizes  in  the  dihexagonal  pyramidal  class  of  the  hexagonal 
system.  Snow  crystals  are  skeleton  crystals  of  great  variety. 
The  frontispiece  shows  a  number  of  microphotographs  of  snow 
crystals. 

H.  =  IK-  Sp.  gr.  =  0.9167. 

Color.  Colorless  to  white,  bluish  in  thick  layers.  Luster, 
vitreous. 

Optical  Properties.  n7(1.313)  =  na(  1.309)  =  0.004.  Optically 
positive. 

Occurrence.     1.  In  the  form  of  snow,  frost,  and  hail. 

2.  On  the  surface  of  rivers,  ponds,  and  lakes. 

3.  In  the  polar  regions. 

4.  In  glaciers. 


OXIDS 


267 


Cuprite,  Cu2O 


Form.  Cuprite  is  found  in  crystals,  in  crystalline  aggregates, 
and  in  fine-grained  masses.  Crystals  are  isometric;  the  common 
forms  are  the  cube  (a),  octahedron  (o),  and  dodecahedron  (d). 
The  habit  is  usually  determined  by  one  of  these  forms.  (Figs. 
423-426.)  Capillary  cuprite  found  in  Arizona  proves  to  be 
elongate  cubes. 

H.  =  3M  to  4.  Sp.gr.  6.0  ±. 

Color.  Dark  red  to  brownish-red.  Translucent  to  opaque. 
Streak,  brownish-red.  Luster,  metallic-adamantine.  Imperfect 
cleavage. 

Optical  Properties.  Isotropic.  n  =  2.85.  Fragments  are  ir- 
regular, translucent  red,  and  dark  between  crossed  nicols. 


FIG.  423. 


FIG.   424. 


FIG.  425. 


FIG.  426. 


Chemical  Composition.  Cuprous  oxid,  Cu2O;  (Cu  =  88.8  per 
cent.).  Iron  oxid  is  the  most  frequent  impurity. 

Blowpipe  Tests.  On  charcoal  fuses  (at  2*^)  to  a  copper 
button. 

Soluble  in  HNOs  to  a  green  solution. 

Distinguishing  Features.  Cuprite  is  recognized  by  the 
isometric  crystals,  adamantine  luster,  and  absence  of  perfect 
cleavage. 

Uses.  Cuprite  is  a  valuable  copper  ore  on  account  of  the  high 
percentage  of  copper.  Bisbee,  Arizona,  is  an  important  locality. 

Occurrence.  1.  In  the  oxidized  zone  of  ore  deposits  asso- 
ciated with  other  copper  minerals,  especially  native  copper. 
Bisbee,  Arizona. 


268        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


HEMATITE  GROUP— HEXAGONAL 

Corundum  and  hematite  form  a  perfect  isomorphous  group, 
although  intermediate  compounds  are  lacking.  With  them  is 
sometimes  placed  ilmenite,  but  it  is  more  properly  considered 
a  ferrous  metatitanite,  FeTiO3.  It  belongs  to  a  different  crystal 
class,  the  trigonal  rhombohedral  class. 

CORUNDUM,  A1203 

Form.  Corundum  is  found  in  rough,  loose  crystals,  in  cleav- 
able  masses  and  disseminated  through  rock  in  small  crystals  or 
grains.  The  crystals  belong  to  the  scalenohedral  class  of  the 


x/ 


FIG.  427. 


FIG.  428. 


FIG.  429. 


hexagonal  system.  Usual  forms:  cfOOOl},  rflOll),  a|1120}, 
rc{2243}.  Interfacial  angles^  cr(0001 :1011)  =  57° _34',  rKlOll: 
IlOl)  =  93°  56',  cri(0001  :  2243)  =  61°  11';  nn(2243  :  4223)  = 
51°  58'.  Habit  prismatic  (Fig.  428),  tabular  (Fig.  427),  and  steep 
pyramidal  (Fig.  429).  The  trigonal  character  is  shown  by  the 
r  faces  at  alternate  vertices  and  by  the  triangular  striations  on  the 
basal  pinacoid  c. 

Cleavage.  There  is  often  parting  parallel  to  c  and  r.  The 
rhombohedral  parting  greatly  resembles  cubic  cleavage  (rr  = 
93°  56'). 

H.  =  9.  Sp.    gr.   4.0  ±. 


'OXIDS  269 

Color.  Bluish-gray  is  the  most  common  color,  but  brown, 
red,  pink,  green,  bright  blue,  and  white  colors  are  not  at  all  rare. 
Usually  translucent.  Luster,  sub-adamantine. 

Optical  Properties.  nT(1.767)  -  wa(1.759)  =  0.008.  Double 
refraction  rather  low.  Fragments  are  irregular  with  first-order 
interference  colors  and  index  of  refraction  greater  than  methylene 
iodid.  Large  deep-colored  fragments  or  small  crystals  are 
pleochroic. 

Chemical  Composition.  Alumina  or  aluminum  oxid,  A12O3; 
(Al  =  52.9  per  cent.).  Emery  is  a  dark-colored  mixture  of 
corundum  with  magnetite,  hematite  or  spinel. 

Blowpipe  Tests.  Infusible.  When  intensely  heated  with 
Co(NO3)2  solution  it  becomes  deep  blue. 

Insoluble    in   acids.     Decomposed    by   fusion   with    KHSO4. 

Distinguishing  Features.  Corundum  is  recognized  by  its 
extreme  hardness  (it  is  often  altered  on  the  exterior  to  soft  mica- 
ceous product) ,  by  its  cleavage,  and  by  its  high  specific  gravity. 

Uses.  Certain  varieties  of  corundum  are  valuable  gems. 
Ruby,  the  transparent  red  corundum,  is  even  more  valuable  than 
diamond.  Sapphire  is  the  blue  transparent  corundum. 
Colorless  stones  are  known  as  white  sapphires.  The  best  rubies 
come  from  Burma,  and  the  best  sapphires  from  Ceylon. 

Artificial  rubies  and  sapphires  are  now  produced  synthetically 
in  Paris.  They  are  with  difficulty  distinguished  from  natural 
stones. 

Corundum  is  also  used  as  an  abrasive,  either  as  the  pure  cleav- 
able  mineral  or  as  the  mixture  known  as  emery.  Corundum  is 
mined  in  Ontario,  Canada,  and  emery  in  Asiatic  Turkey. 

Artificial  corundum  is  now  made  by  heating  bauxite  in  the 
electric  furnace.  It  is  sold  under  the  trade  names  alundum 
and  aloxite. 

Occurrence.  1.  In  certain  igneous  rocks  such  as  syenites  and 
nepheline  syenites  in  which  an  excess  of  A12O3  has  crystallized 
out  as  corundum,  just  as  an  excess  of  SiO2  crystallizes  as  quartz 
in  granites.  Craigmont,  Ontario. 


270        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

2.  In   peridotites  along   the   borders   of   adjacent  rocks.     In 
North  Carolina  the  country  rocks  are  gneisses,  but  the  mode  of 
origin  is  doubtful. 

3.  In  crystalline  limestones  (Burma,  New  York,  New  Jersey). 

4.  In  lamprophyre  dikes,  probably  the  result  of  absorption  of 
shale  and  subsequent  recrystallization  of  the  excess  of  alumina. 
Yogo  Gulch,   Montana.     Emery  is  associated  with  limestone 
at  Naxos,  Greece,  and  perhaps  is  the  metamorphic  equivalent  of 
bauxite. 

5.  In  sands  and  gravels.     The  gem-bearing  gravels  of  Ceylon 
furnish  sapphire  and  other  varieties  of  corundum. 

HEMATITE,  Fe2O3 

Form.     Hematite  is  found  in  a  variety  of  forms :  small  crystals 
in  cavities,   micaceous,   fibrous,   oolitic,   and   massive  compact. 


FIG.  430.  FIG.  431. 

FIGS.  430-432. — Hematite  crystals. 


FIG.  432. 


Some  of  the  red  massive  minerals  called  hematite  are  probably 
turyite,  its  amorphous  equivalent. 

Crystals  are  hexagonal,  usually  tabular  or  low  rhombohedral 
in  habit  (Figs.  430,  431).  The  island_of  Elba  furnishes  good 
crystals  with  the  forms  w{10l4|,  rjlOTl),  and  n{2243|  repre- 
sented in  plan  by  Fig.  432.  cr(0001 :  lOll)  =  57°  37. 

H.  =  6.  Sp.  gr.  5.2±. 

Color.  Iron-black  to  dark  red.  Streak,  brownish-red. 
Luster,  metallic  to  dull.  Opaque,  but  translucent  red  in  very 


OX  IDS  271 

thin  scales.  These  scales  are  dark  between  crossed  nicols  (basal 
sections). 

Chemical  Composition.  Ferric  oxid,  Fe2O3;  (Fe  =  70.0  per 
cent.).  The  iron  is  sometimes  partly  replaced  by  titanium 
and  magnesium. 

Blowpipe  Tests.  Fusible  with  difficulty  (5^).  On  charcoal 
in  R.F.  becomes  magnetic.  Gives  bead  tests  for  iron. 

Slowly  soluble  in  concentrated  HC1. 

Distinguishing  Features.  Hematite  is  distinguished  from 
magnetite,  ilmenite,  and  limonite  by  its  brownish-red  streak  and 
from  turyite  by  its  crystalline  nature,  greater  hardness,  and 
absence  of  water. 

Uses.  Hematite  is  the  principal  ore  of  iron  ;  the  Lake  Superior 
district  furnishes  the  principal  domestic.  supply. 

Occurrence.  1.  In  basic  igneous  rocks  as  a  late  magmatic 
mineral.  Engels  Mine,  Plumas  county,  California. 

2.  In  cavities  of  lavas  as  a  volcanic  sublimate.     Vesuvius. 

3.  In    contact-metamorphic    deposits    often    associated    with 
magnetite  and  pyrite. 

4.  As  a  metasomatic  replacement  of  cherty  iron  carbonate. 
This  origin  is  assigned  to  the  Lake  Superior  hematite. 

5.  In  metamorphic  rocks  often  forming  hematite  schists  and 
quartz-hematite  schists. 

6.  As   an   alteration   product   of   other  iron   minerals.     The 
fibrous  pencil-ore  of  England  is  supposed  to  be  formed  by  the 
dehydration  of  limonite.     Martite  is  a  pseudomorph  of  hema- 
tite after  octahedral  crystals  of  magnetite. 


Turyite, 

Form.  The  typical  occurrence  of  turyite  (formerly  called 
turgite)  is  in  colloform  crusts  usually  with  a  fibrous  structure. 
It  also  occurs  in  massive  forms.  It  is  probably  the  amorphous 
equivalent  of  hematite. 

H.  =  5  to  6.  Sp.  gr.  =  3.5-5.0. 

Color,  black  to  dark  red.     Streak,  dark  cherry-red. 


272        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Optical  Properties,  n,  variable  2.4-2.6.  Fragments  are  thin 
splinters  which  under  crossed  nicols  in  direct  sunlight  show  a 
deep  red  color. 

Chemical  Composition.  Ferric  oxid  with  a  variable  amount  of 
water,  Fe2O3(H20)x-(H2O  usually  =  4  to  6  per  cent.). 

Blowpipe  Tests.  Fusible  with  difficulty.  In  the  closed 
tube  decrepitates  and  gives  water. 

Soluble  in  dilute  HC1. 

Distinguishing  Features.  Turyite  is  distinguished  from  hema- 
tite by  the  presence  of  water,  and  from  goethite  and  limonite 
by  its  red  streak. 

Occurrence.  1.  In  the  oxidized  zone  of  various  ore-deposits, 
often  associated  with  goethite  and  limonite. 

RUTILE  GROUP— TETRAGONAL 

Cassiterite  (SnO2)  and  rutile  (Ti02)  together  with  plattnerite 
(PbO2),  polianite  (MnO2),  zircon  (ZrSiO4  or  ZrO2-SiO2),  and 
thorite  (ThSi04  or  ThO2-Si02)  are  isomorphous;  all  are  tetrag- 
onal dioxids  of  tetravalent  metals. 

CASSITERITE,    SnO2 

Form.  Cassiterite  is  found  in  crystals,  crystalline  and  reni- 
form  masses,  pebbles,  and  grains  (stream-tin).  Crystals  are 
tetragonal  and  prismatic  or  pyramidal  in  habit.  Twins  are 
common. 

H.  =  6>^.  Sp.    gr.    7.0 ±. 

Color,  black  or  brown.     Luster,  adamantine. 

Optical  Properties.  ny  (2.09)  --  na(1.99)  =  0.10.  Double 
refraction  strong.  Fragments  are  irregular  with  high-order 
interference  colors  and  high  relief  even  in  methylene  iodid. 
Some  varieties  are  pleochroic. 

Chemical  Composition.  Tin  oxid,  SnO2;  (Sn  =  78.6  per 
cent.). 

Blowpipe  Tests.  Infusible:  Fused  with  sodium  carbonate, 
sulfur,  and  a  little  powdered  charcoal  gives  a  metallic  button 


OXIDS  273 

and  a  straw-colored  coating  near  the  assay.  The  coating  heated 
with  Co  (NO  3)  2  solution  assumes  a  bluish-green  color.  Placed 
on  zinc  and  treated  with  dilute  HC1,  the  mineral  gives  a  coating 
of  tin,  which  takes  a  good  polish  when  rubbed. 

Insoluble  in  acids. 

Distinguishing  Features.  The  high  specific  gravity  and 
adamantine  luster  serve  to  distinguish  cassiterite  from  other 
minerals. 

Uses.  Cassiterite  is  practically  the  only  source  of  tin.  The 
Malay  States  lead  in  the  production  of  tin,  with  Bolivia  second. 

Occurrence.  1.  In  tin-stone  veins  associated  with  topaz,  wol- 
framite, arsenopyrite,  lepidolite,  and  fluorite.  Granite  is  the 
country  rock.  Zinnwald,  Bohemia,  is  a  prominent  locality. 

2.  In  greisen  (quartz-muscovite  rock)  and  other  rocks  affected 
by  the  intrusion  of  pegmatites,   but  rare  in   the   pegmatites 
themselves. 

3.  In  rhyolites  and  quartz  porphyries  often  accompanied  by 
topaz.     Durango,  Mexico. 

4.  In  sands  and  gravels.     (Stream-tin). 

Rutile,   TiO2 

Form.  Rutile  is  found  in  embedded  grains  or  crystals,  as 
acicular  inclusions  or  in  a  massive  form.  Crystals  are  tetragonal 
and  usually  prismatic  in  habit.  Usual  forms:  p{lll[,  ejlOl), 
a{100),  m{110}.  Interfacial  angles:  pm(l  11  :  100)  =  47°  40'; 
ea(101  :  100)  =  57°  13';  ee(lQl  :  Oil)  =  45°  2'.  Figures  433  to  436 
represent  various  types  of  twinned  crystals  with  e{101}  as  twin- 
plane. 

Cleavage.     Imperfect  prismatic. 

H.  =  6-6^.  Sp.  gr.  4.2  ±. 

Color.  Red,  brownish-red  to  black.  Streak,  pale  brown. 
Luster,  metallic-adamantine. 

Optical  Properties.  ny(2 . 90)  -  na(2 . 62)  -  0 . 28.  Fragments 
are  yellow  and  irregular  with  high-order  interference  colors  and 
high  relief  even  in  methylene  iodid. 

18 


274 


INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Chemical  Composition.  Titanium  oxid,  TiO2;  (Ti  =  60.0  per 
cent.).  Iron  is  usually  present. 

Blowpipe  Tests.  Infusible.  Gives  a  violet  NaPO3  bead  in 
R.F.  (Use  very  fine  powder  and  first  heat  in  O.F.) 

Insoluble  in  acids. 

Distinguishing  Features.  Distinguished  from  cassiterite  by 
lower  specific  gravity.  The  red  color  and  metallic-adamantine 
luster  are  distinctive. 

Uses.  Rutile  is  used  as  coloring  matter  for  porcelain  and  as 
a  source  of  ferro-titanium.  Nelson  County,  Virginia,  is  an  im- 
portant locality. 


FIG.  433. 


FIG.  434. 


FIG.  435. 


FIG.  436. 


Occurrence.  1.  As  a  constituent  of  high-temperature  veins 
(or  pegmatites)  associated  with  apatite  and  scapolite.  The  coun- 
try rock  is  gabbro.  Kragero,  Norway. 

2.  As  a  secondary  mineral  in  various  rocks  such  as  gneisses, 
schists,  and  clays.  The  rutile  is  set  free  by  the  decomposition 
of  titanium-bearing  silicates,  especially  the  pyroxenes.  Rutile 
is  also  an  alteration  product  of  titanite  and  occurs  as  a  paramorph 
after  brookite  (an  orthorhombic  form  of  TiO2). 

Pyrolusite,  MnO2  (H2O), 

Form.  Pyrolusite  occurs  in  fibrous  and  columnar  forms,  in 
acicular  crystals,  in  crusts,  in  masses,  and  along  seams  in  den- 


OXIDS  275 

dritic  forms.     Crystals  are  prismatic  but  indistinct,  and  probably 
always  pseudomorphous  after  manganite. 

H.  =  1  to  2.  Sp.  gr.  4.8  ±. 

Color,  black.  Streak,  black.  Luster,  metallic  to  dull. 
Opaque. 

Chemical  Composition.  Manganese  dioxid  with  a  little  ad- 
sorbed water,  MnO2  (H2O)x;  (Mn  =63.2  per  cent.). 

Blowpipe  Tests.  Infusible.  In  closed  tube  it  gives  a  small 
amount  of  water  (usually  about  2  per  cent.).  Gives  manganese 
bead  tests. 

Soluble  in  HC1  with  the  evolution  of  chlorin. 

Distinguishing  Features.  Distinguished  from  other  manga- 
nese oxids  by  its  inferior  hardness  and  small  water  content. 

Uses.  Pyrolusite  is  one  of  the  prominent  ores  of  manganese, 
but  it  is  usually  mixed  with  psilomelane  or  manganite.  It  is  also 
used  in  the  manufacture  of  chlorin  and  has  other  minor  uses. 

Occurrence.  1.  Pyrolusite  is  probably  in  most  cases  formed 
by  the  dehydration  of  manganite.  Its  occurrence  is  similar  to 
that  of  the  other  manganese  oxids.  Hants  county,  Nova  Scotia. 

Stibiconite,  Sb2O4(H2O)a! 

Form.  Stibiconite  occurs  massive  or  as  a  coating.  It  is 
never  found  crystallized,  but  is  sometimes  pseudomorphous  after 
stibnite. 

H.=4  to  5.  Sp.  gr.  5.2  +  . 

Color,  pale  yellow.    Luster,  dull. 

Optical  Properties.  n  =  1.61-1. 75.  Fragments  are  irregular, 
color  pale  yellow,  and  isotropic. 

Chemical  Composition.  Amorphous  antimony  tetroxid  with 
adsorbed  or  dissolved  water;  Sb2O4(H2O)x  (Sb  about  75  per 
cent.). 

Blowpipe  Tests.     Infusible.     In  the  closed  tube  gives  water. 

Insoluble  in  HC1. 


276        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Distinguishing  Features.  Stibiconite  is  recognized  by  its  pale 
yellow  color  and  high  specific  gravity.  It  is  usually  associated 
with  stibnite. 

Uses.  A  minor  ore  of  antimony  but  abundant  in  certain 
localities. 

Occurrence.  1.  A  secondary  mineral  often  found  with  stibnite 
and  resulting  from  its  oxidation. 


6.  ALUMINATES,  FERRITES,  ETC. 


Spinel, 

MAGNETITE, 

Franklinite, 

CHROMITE, 

Hausmannite 


MgAl2O4 
FeFe2O4 
(Zn,Mn)Fe2O4 

(Fe,Mg)(Cr,Al)204 
Mn3O4 


These  minerals  are  sometimes  considered  as  oxids,  but  they  are 
probably  salts  of  certain  unfamiliar  acids.  Spinel  is  magnesium 
metaluminate  derived  from  HA102(H3A1O3  —  H2O).  Magnetite 
is  ferrous  metaferrite;  iron  acts  both  as  an  acid  and  as  a  base. 
Chromite  is  essentially  ferrous  metachromite,  derived  from 
HCr02  (HaCrOa  -  H2O). 


SPINEL  GROUP—  ISOMETRIC 

Four  of  the  enumerated  minerals  belong  to  the  spinel  group, 
which  is  one  of  the  best  known  examples  of  isomorphism,  for 
many  intermediate  compounds  exist.  The  minerals  of  this  group 
are  isometric  and  usually  crystallize  in  octahedrons.  Besides  the 
minerals  mentioned  there  are  also  hercynite  (FeA^O^,  gahnite 
(ZnAl2O4),  and  jacobsite  (MnFe2O4).  The  general  formula,  then, 
is:  (Mg,Fe,Mn,Zn)(Al,Fe,Cr,Mn)2O4.  The  following  analyses 
illustrate  the  range  and  variation  in  composition. 

Analyses  of  Minerals  of  the  Spinel  Group 


MgO 

FeO 

MnO 

ZnO 

A12O3 

Fe2Os 

Mn2O3 

CrzOs 

Misc. 

Spinel  

24.6 

4.6 

69.7 

1.6 

Spinel    pleonaste) 

19.9 

11.6 

68  5 

Spinel  (picotite).  . 

23.6 

3.9 

53.9 

11.4 

7.2 

Hercynite  

2.9 

35.7 

61.2 

Magnetite  

3.0 

26.1 

tr 

70.6 

TiOz  =  0.3 

Magnetite  

2.1 

27.7 

0.4 

1.1 

68.5 

0.6 

Gahnite  

0.1 

1.1 

39.6 

49.8 

Si02  =  0.6 

Franklinite  

10.5 

23.1 

63.4 

4.4 

Jacobsite  

6.4 

20.7 

68.3 

4.0 

Chromite  

4.4 

25.0 

0.9 

7.2 

59.2 

SiO2  =  3.2 

Chromite  

14.1 

18.0 

0.5 

12.1 

56.5 

277 


278        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Spinel,  MgAl2O4 

Form.  Spinel  is  practically  always  found  in  crystals  or  grains, 
usually  disseminated,  but  sometimes  loose  in  sands  and  gravels. 
Crystals  are  isometric,  the  octahedron  being  the  only  common 
form  (Fig.  437).  Contact-twins  with  {111}  as  twinning  plane 
are  so  common  that  this  twin-law  is  known  as  the  spinel  law 
(Fig.  438). 

H.  =  8.     Sp.  gr.  3.6  to  4.0,  depending  upon  composition. 

Color,  black  and  dark  shades  of  gray,  brown,  and  green;  also 
red  and  blue.  Usually  translucent.  Luster,  sub-adamantine. 

Optical  Properties.  Isotropic.  n  =  1.72.  Fragments  are  ir- 
regular and  dark  between  crossed  nicols.  The  usual  color  of  the 
fragments  is  green  (pleonaste)  and  coffee-brown  (picotite). 


FIG.  437. 


FIG.  438. 


Chemical  Composition.  Magnesium  metaluminate,  MgAl204 
or  MgOAl203;  (MgO  =  28.2  per  cent.).  The  magnesium  is  of  ten 
replaced  by  ferrous  iron,  and  the  aluminum  by  chromium  and 
ferric  iron.  The  iron-bearing  spinel  is  called  pleonaste  and  the 
chrome-bearing  spinel,  picotite. 

Blowpipe  Tests.  Infusible,  but  the  color  may  change  on  heat- 
ing. Turns  blue  when  heated  with  cobalt  nitrate  solution. 

Insoluble  in  hydrochloric  and  nitric  acids.  Decomposed  by 
fusion  with  potassium  acid  sulfate. 

Distinguishing  Features.  Distinguished  from  most  minerals 
by  its  octahedral  form  and  superior  hardness.  The  black 
variety  is  distinguished  from  magnetite  by  its  non-metallic  luster 


ALUMINATES,  FERRITES,  ETC.  279 

and  non-magnetic  character.  From  ruby,  the  red  variety  is 
distinguished  by  optical  tests. 

Uses.     A  red  variety  called  spinel-ruby  is  used  as  a  gem. 

Occurrence.  1.  As  a  contact  mineral  in  crystalline  limestone 
associated  with  phlogopite,  chondrodite,  corundum,  and  graphite. 
Amity,  New  York,  is  a  prominent  locality. 

2.  As  an  accessory  mineral  in  various  igneous  and  meta- 
morphic  rocks.     Pleonaste  occurs  with  emery,  and  picotite,  with 
serpentine. 

3.  In  gem-bearing  gravels.     (Ruby  spinel.)     Ceylon. 

MAGNETITE,  FeFe2O4  (or  Fe3O4) 

Form.  Magnetite  occurs  in  loose  and  attached  crystals,  in 
compact  and  granular  masses,  and  in  the  form  of  sand.  Crystals 


FIG.  439.  FIG.  440.  FIG.  441. 

belong  to  the  hexoctahedral  class  of  the  isometric  system.  The 
only  common  forms  are  the  octahedron  o,  the  dodecahedron  d, 
and  the  trapezohedron  raj  311}.  The  habit  is  octahedral,  more 
rarely  dodecahedral,  but  almost  never  cubic.  Figures  439,  440, 
and  441  represent  typical  crystals. 

Cleavage.     Some  specimens  have  octahedral  parting. 

H.  =  6.  Sp.  gr.    5.1  ±. 

Color,  black.  Streak,  black.  Opaque.  Luster  metallic. 
Strongly  attracted  by  the  magnet  and  sometimes  is  a  magnet 
itself  (lodestone). 


280        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Chemical  Composition.  Ferrous  metaferrite,  FeFe204  or 
Fe3O4.  (FeO  =  31.0;  Fe  =  72.4  per  cent.).  May  contain 
magnesium,  manganese,  or  titanium. 

Blowpipe  Tests.  Fusible  with  difficulty  (5^).  Gives  bead 
tests  for  iron. 

Soluble  in  concentrated  HC1.  The  hydrochloric  acid  solution 
of  the  borax  fusion  gives  tests  for  both  ferrous  and  ferric  iron. 

Distinguishing  Features.  Magnetite  is  distinguished  from  all 
other  black  minerals  by  its  strong  magnetism.  Hematite  and 
chromite  are  often  similar  but  are  recognized  by  differences  in 
streak. 

Uses.  Magnetite  is  an  important  ore  of  iron,  mined  in  New 
York,  New  Jersey,  and  Pennsylvania,  and  in  Scandinavia  it  is  the 
principal  iron  ore. 

Occurrence.  1.  A  very  common  and  widely  distributed 
accessory  constituent  of  igneous  rocks. 

2.  In  ore-deposits  due  to  magmatic  segregation  at  the  end  of 
the  magmatic   period.     The   Scandinavian   magnetite  has  this 
origin. 

3.  As  a  contact  mineral  between  igneous  rocks  and  limestones 
often  occurring  with  pyrite  and  hematite. 

4.  In  lenses  and  layers  in  schists  and  gneisses. 

5.  As  an  alteration  product  of  iron-bearing  silicates  in  more  or 
less  altered  igneous  rocks  (serpentines,  for  example). 

6.  In  detrital  deposits  as  the  main  constituent  of  the  so-called 
black  sands  which  are  prominent  on  the  Pacific  Coast. 

Franklinite,  (Zn,Mn)Fe2O4 

Form.  Franklinite  occurs  in  disseminated  crystals  or  in  granu- 
lar aggregates.  The  crystals  are  usually  octahedrons,  modified 
by  the  dodecahedron  (like  Fig.  426,  p.  267). 

H.  =  6.  Sp.  gr.  5.1±. 

Color,  black.  Opaque.  Luster,  metallic.  Streak,  dark 
brown.  Slightly  magnetic. 

Chemical    Composition.     Zinc    and    manganese    metaferrite, 


ALUMINATES,  FERRITES,  ETC.  281 

(Zn,Mn)Fe2O4  or  (Zn,Mn)O-Fe2O3.  Some  analyses  show  fer- 
rous iron  and  manganic  manganese.  A  typical  analysis  is  given 
on  p.  277. 

Blowpipe  Tests.  Infusible.  In  O.F.  the  borax  bead  is  ame- 
thyst (Mn),  while  in  R.F.  it  is  green  (Fe).  On  charcoal  with 
sodium  carbonate  it  gives  a  white  coating  of  ZnO  and  a  magnetic 
residue. 

Soluble  in  HC1  with  the  evolution  of  a  little  chlorin. 

Distinguishing  Features.  Franklinite  resembles  magnetite 
and  chromite,butmay  usually  be  distinguished  by  its  association 
with  willemite  and  zincite  (a  dark  red  mineral  with  the  composi- 
tion: ZnO). 

Uses.  Franklinite,  extensively  mined  in  Sussex  county, 
New  Jersey,  is  used  for  the  production  of  zinc  white.  The 
residue  is  used  for  the  production  of  spiegeleisen,  an  iron-manga- 
nese alloy. 

Occurrence.  1.  In  crystalline  limestone  with  willemite, 
zincite,  and  rhodonite.  Sussex  County,  New  Jersey,  is  practi- 
cally the  only  locality  for  this  mineral.  This  deposit  was  prob- 
ably formed  by  the  metamorphism  of  a  sedimentary  limestone 
containing  calamine  and  some  manganese  mineral. 

CHROMITE,  (Fe,Mg)(Cr,Al)204 

Form.  Chromite  occurs  disseminated  and  in  compact  masses, 
rarely  in  small  octahedral  crystals. 

H.  =  5^.  Sp.  gr.  4.4  ±. 

Color,  black.  Streak,  dark  brown.  Luster,  submetallic  or 
metallic.  Opaque.  Some  varieties  are  slightly  magnetic  on 
account  of  the  presence  of  the  magnetite  molecule  in  solid 
solution. 

Optical  Properties.  Isotropic  n>1.93.  Thin  fragments  are 
irregular,  usually  translucent  brown,  and  dark  between  crossed 
nicols. 

Chemical  Composition.     Ferrous  and  magnesium  metachro- 


282        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

mite  and  metaluminate.  Ferric  iron  may  also  be  present.  Two 
typical  analyses  are  given  on  page  277. 

Blowpipe  Tests.  Infusible.  Gives  chromium  bead  tests. 
Fused  with  sodium  carbonate  it  gives  a  magnetic  mass. 

Insoluble  in  acids.  Decomposed  by  sodium  carbonate  with 
the  formation  of  sodium  chromate,  which  is  soluble  in  water. 

Distinguishing  Features.  The  submetallic  luster  is  distinc- 
tive. It  is  usually  associated  with  antigorite. 

Uses.  Chromite  is  the  only  source  of  the  salts  of  chromium 
such  as  potassium  chromate,  potassium  dichromate,  and  lead 
chromate.  Chromite  bricks  are  used  as  a  furnace-lining  for 
certain  kinds  of  smelting.  Ferro-chrome  is  an  alloy  used  in 
making  chrome-steel.  New  Caledonia  and  Rhodesia  are  the 
principal  sources  of  chromite.  Important  deposits  exist  in 
California. 

Occurrence.  In  peridotites  and  derived  serpentines  as  an 
original  or  residual  constituent.  Ore  deposits  may  be  due  to 
magmatic  segregation.  Woods  mine,  Lancaster  county,  Penn- 
sylvania. 

2.  In  serpentines,  probably  derived  from  chromium-bearing 
olivine  in  the  process  of  the  serpentinization  of  peridotite.  Lake 
county,  California. 


Hausmannite,  Mn3O4 

Form.  Hausmanite  is  usually  a  massive  mineral  but  some- 
times it  occurs  in  euhedral  tetragonal  crystals  with  the  tetra- 
gonal bipyramid  {111}  as  the  dominant  form. 

Cleavage,  fairly  distinct. 

H  =   5  to  5}^.  Sp.gr.  4.8 ±. 

Color,  steel  gray  to  brownish,  reddish  black.  Streak,  chestnut 
brown.  Luster  submetallic. 

Optical  Properties  n7(2.45)  -  w«(2.15)  =  0.30  (Larsen). 
Fragments  are  dark  red  and  doubly  refracting  when  examined 
in  direct  sunlight  between  crossed  nicols. 


ALUMINATES,  FERR1TES,  ETC.  283 

Chemical  Composition,  probably  a  manganese  manganite, 
MnMn2O4  (=  Mn3O4).  Mn  =  72  per  cent. 

Blowpipe  Tests.     Infusible.     It  gives  the  Mn  bead  tests. 

Soluble  in  HC1  with  the  evolution  of  chlorin. 

Distinguishing  Features.  It  is  distinguished  from  the  man- 
ganese dioxide  minerals  by  its  streak  and  absence  of  water. 

Uses.  Hausmannite  is  one  of  the  minor  ores  of  manganese. 
It  has  been  mined  at  Batesville,  Arkansas. 

Occurrence.  1.  A  hydrothermal  replacement  of  limestone. 
Often  altered  to  psilomelane. 


7.  HYDROXIDS 


Goethite, 

Fe2O3H2O 

Manganite, 

Mn2O3H20 

LIMONITE, 

H2Fe204(H20)x 

Gibbsite, 

A1(OH)3 

CL1ACHITE, 

Al2O3(H2O)a; 

Brucite, 

Mg(OH)2 

PSILOMELANE,  4MnO2  (Ba,K2)O  -(H2O)*(?) 

The  hydroxids  or  hydrous  oxids  are  in  part  normal  hydroxids 
such  as  Mg(OH)2.  Others  may  be  derived  by  subtracting 
water  from  the  normal  compounds.  For  example,  2Fe(OH)3  — 
2H20  =  Fe2O3-H2O,  goethite. 

Goethite,  Fe2O3H2O 

Form.  Goethite  is  found  in  small  acicular  crystals,  in  bladed 
crystal  aggregates,  and  in  scaly  or  fibrous  masses.  Crystals  are 
orthorhombic,  but  are  usually  too  minute  to  decipher. 

Cleavage,  in  one  direction  parallel  to  the  length. 

H.  =  5J£.  Sp.gr.  4.3  ±. 

Color,  yellowish-brown  to  nearly  black.  Streak,  yellowish- 
brown  like  that  of  limonite.  Luster,  metallic-adamantine. 

Optical  Properties.  n7(2.40)  -  rca(2.26)  =  0.14.  Thin 
fragments  are  prismatic,  and  translucent  brown  with  parallel 
extinction. 

Chemical  Composition.  Ferric  oxid  monohydrate,  Fe2O3-H2O 
(H2O  =  10.1  percent.).  Itusually  contains  a  little  manganese. 
Fibrous  varieties  contain  an  excess  of  water  over  that  required 
by  the  formula. 

Blowpipe  Tests.  Fusible  with  difficulty  (5^).  In  the  closed 
tube  turns  red  and  gives  off  water.  Bead  tests  for  iron. 

Soluble  in  HC1. 

284 


HYDROXIDS  285 

Distinguishing  Features.  The  yellow-brown  streak  and 
fibrous  or  bladed  structure  are  distinctive.  It  is  distinguished 
from  limonite  by  the  fact  that  it  is  crystalline. 

Uses.  As  an  ore  of  iron  it  is  classed  as  brown  hematite  along 
with  limonite. 

Occurrence.  1.  In  iron-ore 'deposits  along  with  limonite  and 
hematite. 

2.  As  inclusions  in  various  minerals  such  as  feldspars,  quartz, 
etc. 

Manganite,  Mn2O3  H2O 

Form.  Manganite  is  found  in  prismatic  crystals  and  in  colum- 
nar and  fibrous  masses.  Crystals  are  orthorhombic,  prismatic 
in  habit,  and  striated  parallel  to  their  length. 

Cleavage,  in  one  direction  (010)  parallel  to  the  length  of 
the  crystals. 

H.  =  4.  Sp.gr.  4.3  ±. 

Color.  Iron-black  or  dark  gray .  Streak,  dark  brown.  Luster, 
submetallic.  Opaque. 

Chemical  Composition.  Manganic  oxid  monohydrate  Mn2O3-- 
H2O;  (H20  =  10.3  per  cent.). 

Blowpipe  Tests.  Infusible.  In  the  closed  tube  gives  water. 
Bead  tests  for  manganese. 

Soluble  in  HC1  with  the  evolution  of  chlorin. 

Distinguishing  Features.  Manganite  is  distinguished  from 
psilomelane  by  its  crystalline  structure  and  inferior  hardness, 
and  from  pyrolusite  by  its  greater  hardness,  brown  streak,  and 
higher  water  content. 

Uses.  Manganite  is  an  ore  of  manganese  occurring  along  with 
pyrolusite  and  psilomelane. 

Occurrence.  1.  As  a  vein  mineral.  Ilefeld  in  the  Harz  Mts. 
is  a  prominent  locality. 

2.  As  a  secondary  mineral  in  residual  clays  associated  with 
psilomelane. 


286        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

LIMONITE,  H2Fe2O4(H20)x 

Form.  Limonite  occurs  in  colloform  crusts  and  in  compact, 
pisolitic,  nodular,  porous,  and  earthy  masses.  It  is  often  pseudo- 
morphous  after  other  iron  minerals,  especially  pyrite. 

H.  =  5%.  Sp.  gr.  3.8  ±. 

Color,  yellow,  brown,  or  black.  Streak,  yellowish-brown. 
Luster,  submetallic  to  dull. 

Optical  Properties,  n  =  2.0  —  2.1.  Fragments  are  ir- 
regular, and  either  iso tropic  or  doubly  refracting. 

Chemical  Composition.  Ferric  hydroxid,  H2Fe2O4(H20)a; 
(Fe=  50  to  60  per  cent.).  (H2O  =  13  to  18  per  cent.).  Often 
impure  from  the  presence  of  manganese  oxid,  phosphates,  clay, 
sand,  and  organic  matter. 

Blowpipe  Tests.  Fusible  with  difficulty  (5^).  When  heated 
in  R.F.  it  becomes  magnetic.  In  the  closed  tube  it  turns  red  and 
yields  water.  Iron  bead  tests. 

Soluble  in  HC1. 

Distinguishing  Features.  Limonite  is  distinguished  from 
hematite  by  its  red  streak  and  from  goethite  by  the  absence  of 
fibrous  structure  and  by  its  optically  isotropic  character. 

Uses.  Limonite  is  a  prominent  ore  of  iron  and  in  the  United 
States  ranks  next  to  hematite  in  importance. 

Occurrence.  1.  As  a  secondary  mineral  in  veins  and  ore 
deposits  formed  by  the  oxidation  of  pyrite.  It  constitutes  an 
important  part  of  the  gossan  or  "iron-hat." 

2.  As  a  metasomatic  replacement  of  limestone. 

3.  As   sedimentary   bedded   deposits,    often   with   an   oolitic 
structure  and  perhaps  formed  from  original  oolitic  siderite.     The 
"minette"  ores  of  Lorraine  and  Luxembourg  belong  here. 

4.  As  bog  iron  ore  formed  by  the  oxidation  of  FeH2(CO3)2  in 
solution  in  marshes  (probably  by  iron  bacteria). 

5.  As  a  pigment  and  stain  in  various  rocks. 

Gibbsite  (Hydrargillite)  A1(OH)3 

Form.  Gibbsite  occurs  in  minute  pseudohexagonal  crystals, 
often  lining  cavities  and  in  stalactitic  and  incrusting  forms.  In 


HYDROXIDS  287 

thin  sections  it  is  sometimes  seen  as  a  crystalline  aggregate 
pseudomorphous  after  feldspars. 

Cleavage  in  one  direction. 

H.  =  2^  to  3K-  SP-  gr-  2.4  ±. 

Color.     Colorless,  white,  gray,  and  pale  colors. 

Optical  Properties.  71/1.558)  -  wtt(1.535)  =  0.023.  Frag- 
ments are  platy  or  prismatic  to  acicular  with  oblique  extinction. 
The  inference  colors  vary  from  first-order  gray  up  to  lower  second- 
order. 

Chemical  Composition.  Aluminum  hydroxid,  A1(OH)3;  (H2O 
=  34.6  per  cent.). 

Blowpipe  J^ests.  Infusible.  When  heated  with  cobalt  nitrate 
solution  it  becomes  blue.  In  the  closed  tube  yields  water. 

Insoluble  in  dilute  HC1. 

Uses.  Gibbsite  is  one  of  the  constituents  of  bauxite,  which  is 
used  as  a  source  of  aluminum  and  aluminum  salts. 

Distinguishing  Features.  Some  varieties  resemble  chalce- 
dony from  which  it  is  distinguished  by  inferior  hardness.  Gibb- 
site is  crystalline,  while  cliachite  is  amorphous. 

Occurrence.  1.  Occurs  along  with  cliachite  in  bauxite,  a 
rock  produced  from  clay  by  desilication.  Bauxite,  Arkansas. 

2.  Occurs  in  limonitic  iron  ores.  Clove  mine.  Dut chess  county, 
New  York. 

CLIACHITE,  A12O3  (H,O)  * 

Forms.  Cliachite  is  an  amorphous  mineral  which  is  the  prin- 
cipal constituent  of  the  rock  called  bauxite.  It  occurs  in  pisolitic 
forms,  more  rarely  in  clay-like  masses.  The  crystalline  equiva- 
lent of  cliachite  is  gibbsite  (A1(OH)3). 

H.  -  1  to  3.  Sp.  gr.  2.5 ±. 

Color,  white,  yellowish- white,  pale  red,  or  brownish-red.  Lus- 
ter, dull  and  earthy. 

Optical  Properties,  n  about  1.57.  Fragments  are  irregu- 
lar and  isotropic.  Associated  doubly-refracting  particles  are 
usually  gibbsite. 


288        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Chemical  Composition.  Hydrous  aluminum  oxid,  A1203- 
(H20)x.  Cliachite  often  contains  iron  (up  to  15  per  cent.). 

Blowpipes  Tests.  Infusible.  Heated  intensely  with  cobalt 
nitrate  solution  it  becomes  blue.  In  the  closed  tube  gives 
abundant  water.  If  pure,  it  completely  dissolves  in  the  NaP03 
bead,  while  clay,  on  the  other  hand,  leaves  a  residue  of  silica. 

Soluble  in  HC1  with  difficulty. 

Distinguishing  Features.  Cliachite  is  easily  recognized  on 
account  of  the  pisolitic  structure.  The  clay-like  variety  can  only 
be  distinguished  by  proving  the  absence  of  silica. 

Uses.  Bauxite  is  now  practically  the  only  ore  of  aluminum. 
It  is  mined  in  Georgia,  Alabama,  Arkansas,  and  Tennessee. 
Bauxite  is  also  used  in  the  production  of  aluminum  salts, 
aluminum  oxid  (used  as  an  abrasive),  and  for  bauxite  bricks. 

Occurrence.  1.  Cliachite  is  the  principal  constituent  of 
bauxite,  a  rock  produced  by  the  desilication  of  clays,  which  in 
Arkansas  were  formed  from  nepheline  syenites. 

Brucite,  Mg(OH)2 

Form.  Brucite  is  occasionally  found  in  crystals,  but  more 
often  in  foliated  masses  and  sometimes  in  fibrous  seams.  Crys- 
tals are  hexagonal  and  tabular  in  habit  with  the  basal  pinacoid 
and  several  rhombohedrons. 

Cleavage,  in  one  direction  parallel  to  (0001). 

H.  =  2M-  Sp.  gr.  ±2.4. 

Color,  white  or  greenish- white.     Luster,  pearly  or  silky. 

Optical  Properties.  nT(1.58)  -na(1.56)  =  0.02.  Cleavage  flakes 
give  a  positive  uniaxial  interference  figure  without  rings  unless 
very  thick.  The  fibrous  variety  gives  acicular  fragments  with 
parallel  extinction  and  negative  elongation. 

Chemical  Composition.  Magnesium  hydroxid,  Mg(OH)2  or 
MgOH2O;  (H2O  =  31.0  per  cent.).  It  often  contains  iron  and 
manganese.  The  manganese  is  due  to  the  isomorphous  replace- 
ment of  Mn(OH)2.  The  latter  occurs  as  a  mineral  called 
pyrochroite. 


HYDROXIDS  289 

Blowpipe  Tests.  Infusible,  but  glows.  Heated  with  cobalt 
nitrate  solution  it  turns  pink.  In  the  closed  tube  yields  water 
and  becomes  opaque. 

Soluble  in  HC1. 

Distinguishing  Features.  Distinguished  from  gypsum  by  its 
fusibility  and  absence  of  calcium.  '  Its  occurrence  is  characteristic. 

Occurrence.  1.  As  a  secondary  mineral  in  serpentine  often 
associated  with  magnesite  and  dolomite.  Texas,  Pennsylvania, 
is  a  prominent  locality. 

2.  In  crystalline  limestones  associated  with  chondrodite  and 
spinel.  It  has  been  produced  by  the  hydration  of  periclase 
(MgO).  Crestmore,  Riverside  County,  California. 


PSILOMELANE,  4MnO2  (Ba,K2)O  - 

Form.  Amorphous.  Psilomelane  is  usually  a  compact  mas- 
sive or  earthy  mineral  without  any  hint  of  crystalline  structure. 
Occasionally  it  shows  a  colloform  surface  in  open  spaces. 

H.  =  3  to  6.  Sp.  gr.  3.0-4.5. 

Color,  black  to  brown.  Streak,  brownish-black  to  brown. 
Luster,  submetallic  to  dull.  Opaque. 

Chemical  Composition.  Impure  hydrous  manganese  dioxid, 
perhaps  4MnO2-(Ba,K2)O-H2O-;  (MnO2  =  70  to  90  per  cent. 
H2O  =  3  to  9  per  cent.).  It  usually  contains  barium  and  potas- 
sium and  sometimes  lithium,  cobalt,  copper,  or  iron. 

Blowpipe  Tests.  Infusible.  In  the  closed  tube  gives  water 
and  also  oxygen.  Manganese  bead  tests. 

Soluble  in  HC1  with  the  evolution  of  chlorin. 

Distinguishing  Features.  Distinguished  from  pyrolusite 
and  manganite  by  the  absence  of  crystalline  structure  and  from 
limonite  by  the  streak. 

Uses.  Psilomelane  is  an  important  ore  of  manganese  and  is 
also  used  as  a  source  of  chlorin.  An  earthy  cobalt-bearing  va- 
riety (asbolane)  is  used  as  a  source  of  cobalt  compounds. 

Occurrence.  1.  In  residual  clays  formed  during  the  process 
of  weathering.  Batesville,  Arkansas. 

2.  In  bog  deposits  often  associated  with  limonite. 

19 


8.  CARBONATES 


A.  Normal  Anhydrous  Carbonates 

CALCITE,    *  CaC03 

DOLOMITE,  CaMg(CO3)2 

Calcite      J  Magnesite,  MgCO3 

Group         SIDERITE,  FeCO3 

Rhodochrosite,  MnCO3 

SMITHSONITE,  ZnCO3 

Aragonite,  CaCO3 

Aragonite      J  Strontianite,  SrCO3 

Group        ]  Witherite,  BaCO3 

CERUSSITE,  PbCO3 

B.  Basic  Carbonates 
MALACHITE,  Cu2(OH)2CO3 

Azurite,  Cu3(OH)2(CO3)2 

Hydromagnesite,        Mg4(OH)2(CO3)3-3H2O 

The  carbonates  are  not  many  in  number,  but  they  include  some 
of  the  most  common  minerals  with  which  the  mineralogist  has 
to  deal.  All  the  important  normal  carbonates  fall  into  two  well- 
defined  isomorphous  groups:  the  calcite  group  (rhombohedral) 
and  the  aragonite  group  (orthorhombic).  These  two  groups 
are  said  to  be  isodimorphous,  as  calcite  and  aragonite  are 
dimorphous. 

CALCITE  GROUP— HEXAGONAL 

The  calcite  group  of  rhombohedral  carbonates  is  a  well  charac- 
terized group  of  familiar  minerals.  These  minerals  crystallize  in 
rhombohedral  and  scalenohedral  crystals  with  cleavage  parallel 
to  the  faces  of  a  rhombohedron  of  about  75°.  All  except  dolo- 
mite belong  to  the  ditrigonal  scalenohedral  class  of  the  hexago- 
nal system.  Dolomite  belongs  to  the  rhombohedral  class,  but 
is  similar  to  the  other  minerals  in  angles  and  other  properties.  All 
the  minerals  of  this  group  are  uniaxial  and  optically  negative,  and 

290 


CARBONATES 


291 


have  very  strong  double  refraction.  Many  isomorphous  mix- 
tures are  known;  and  some  of  them  have  received  special  names 
(see  breunnerite,  ankerite,  and  mesitite  below). 

The   following   analyses   are   representative   of   the   minerals 
mentioned  and  illustrate  isomorphism. 


CaO 

MgO 

FeO 

MnO 

ZnO 

CO2 

Misc. 

Calcite 

56.0 

0.4 

43.5 

H2O  =  0.1 

Calcite         

48.7 

0.9 

0.4 

6.8 

0.4 

40.8 

H2O  =  0.3 

Dolomite  

31.4 

21.2 

47.7 

Dolomite  

29.6 

17.6 

'6.7 

45.6 

Dolomite  (Ankerite)  .  .  . 

28.4 

10.2 

17.2 

44.2 

Magnesite  

.... 

47.3 

0.8 

51.5 

H2O  -  0.5 

Magnesite  (Breunnerite) 

.... 

41.8 

6.5 

0.6 

50.3 

Magnesite  (Mesitite)  .  . 

1.3 

28.1 

24.2 

45.8 

Siderite  

0.2 

59.6 

1.9 

38.0 

Siderite  

2.4 

50.4 

7.5 

38.6 

gangue  =  0.3 

Rhodochrosite  

0.6 

0.4 

0.4 

59.9 

38.3 

Smithsonite  

0.4 

0.1 

64.1 

34.7 

CdO  =  0.6;  CdS  =  0.3 

CALCITE,  CaCO3 

Calcite  has  played  a  very  prominent  part  in  the  history  of 
mineralogy.  The  discovery  of  cleavage  in  calcite  led  to  the 
establishment  of  crystallography  as  an  exact  science  by  Hatiy, 
and  the  discovery  of  double  refraction  in  calcite  led  to  the  de- 
velopment of  crystal  optics.  The  invention  of  the  Nicol  prism, 
which  is  made  of  calcite,  has  made  possible  the  identification  of 
fine-grained  mineral  aggregates  and  rocks. 

Form.  Calcite  is  found  in  well  defined  crystals  (often  large  in 
size),  in  crystalline  crusts  and  druses,  in  cleavable  masses,  in 
various  imitative  forms,  such  as  stalactitic,  pisolitic,  and  oolitic, 
in  granular  masses,  and  sometimes  in  fibrous  forms. 

Calcite  is  the  type  example  of  the  ditrigonal  scalenohedral 
class  of  the  hexagonal  system.  In  number  of  forms  and  variety 
of  their  combinations,  calcite  is  unsurpassed  among  minerals. 
Over  300  well  established  forms,  most  of  them  rhombohedrons 
and  scalenohedrons,  are  known.  6  =  0.854. 


292        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Usual  forms  (in  order  of  their  abundance) :  m{  1010 j ,  c{0001 } , 
e{01JL2),/{0221},  r{10Tl},  M {4041},  w{2131},  a{1120),  2/J3251), 
<{2134). 

Interfacial  angles:  ee(OlT2:I012)  =  45°_3';  em{Oll2:10lO}  = 
63°  45';_rr(1011:1101)  =  74°  55';  m(1011:10lO)  =  45°  23%'; 
.#(0221:  2021)  =  101°  9';  fm (022 1:0 110)  =  26°  53';  MM(4041: 
4401)  =  114°  10/;_Mm(4041:10TO  =  14°_13';  jw(2131:23Tl)  = 
75°_22/;_w(2131:3121)  =  35° _36' ; jwf 213 1:1231)  =  47°  I'^rv 
(1011:2131)  =  29°_6K^mz;(1010:2131)  =  28°  4' ;^2/(325 1:5231) 
=  45°  32^;  3/^(3251:3521)  =  70°  59';  «(?134:2314)  =  41°  55'; 
#(2134:3124)  =  20°  36>^';  mm(10lO:OlTO)  =  60°  0';  ma(10TO: 
1120)  =  30°  0'. 


FIG.  442. 


FIG.  443. 


FIG.  444. 


FIG.  445. 


Figures  442-461  represent  typical  calcite  crystals.  These 
figures  illustrate  the  great  variation  in  habit.  The  habit  is 
variable.  Figures  442-445  are  simple  forms.  Figure  442, 
e{OlT2),  is  an  obtuse  rhombohedron,  while  Fig.  443,  /{0221}, 
is  an  acute  rhombohedron.  Figure  445  is  a  pseudo-cubic  rhom- 
bohedron /i{0332)  with  the  angle  hh  (0332:3302)  =  91°  42r. 
The  dotted  lines  in  each  case  represent  the  cleavage  which  is 
a  great  help  in  orienting  a  crystal.  The  unit  rhombohedron 
rjlOllj  alone  is  rare,  but  it  is  very  frequently  the  dominant 
form  as  in  Figs.  446  and  447.  The  combination  em  (Figs. 
448  and  449)  is  said  to  be  the  most  frequent  combination. 
Figures  450-453  are  common  types.  The  bottom  of  Fig.  451 


CARBONATES 


293 


FIG.  446. 


FIG.  450. 


FIG.  454. 


m 


FIG.  447.  FIG.  448.  FIG.  449. 


FIG.  451. 


FIG.  452.  FIG.  453. 


FIG.  455.  FIG.  456. 


FIG.  458.  FIG.  459.  FIG.  460. 

FIGS.  446-461. — Calcite  crystals. 


FIG.  457. 


FIG.  461. 


294        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


represents  cleavage.  The  faces  of  e  { 01 12 }  are  very  often  striated 
as  in  Fig.  453.  Figures  454  to  457  represent  plans  of  common 
types  of  calcite  crystals.  The  trigonal  symmetry  is  apparent. 
Five  twinning  laws  are  known  for  calcite.  (1)  {0001}  as 
twin-plane.  Figure  458  represents  a  scalenohedron  twinned 
according  to  this  law.  (2)  {01 12)  as  twin-plane.  This  is  of  ten 
polysyn thetic  twinning  with  striations  parallel  to  the  long  diagonal 
(Fig.  459).  3.  {lOll}  as  twin-plane.  Figure  460  represents  the 
combination  {lOTOj,  {Oll2}  twinned  according  to  this  law.  The 
vertical  axes  of  the  two  parts  of  ths  crystal  are  almost  at  right 
angles.  (4)  {0221}  as  twin-plane.  This  is  represented  by  Fig. 
461.  (5)  {2021}  as  twin-plane.  This 
twinning  law  is  very  rare. 

Cleavage,  perfect  rhombohedral  in  three 
directions  at  angles  of  74°  55'.  There  is 
often  parting  parallel  to  {Oll2}  and  this  is 
sometimes  better  developed  than  the 
cleavage  itself. 

H.  =  3.  Sp.  gr.  2.72  ±. 

Color.     Calcite  is  usually  colorless, 
FIG.  462.— cleavage  frag-  white   or  amber,  but  may  be  any  color. 

ments  of  calcite.  .  J 

Luster,  vitreous. 

Optical  Properties.  w7(  1.658)  -  na(1.486)  =  0.172.  The  strong 
double  refraction  is  one  of  the  most  prominent  characters  of 
calcite.  It  may  be  observed  in  Iceland  spar,  the  clear  trans- 
parent cleavable  variety.  Fragments  are  rhombic  (Fig.  462) 
with  symmetrical  extinction  and  very  high-order  interference 
colors.  The  rhombs  often  have  striations  parallel  to  the  long 
diagonal.  These  are  due  to  polysyn  thetic  twinning  produced  by 
pounding  the  fragments.  The  relief  varies  with  the  direction. 
As  shown  in  Fig.  462,  the  rhombs  have  a  high  relief  when  the  long 
diagonal  is  parallel  to  the  vibration  plane  of  the  lower  nicol. 
It  gives  the  microchemical  gypsum  test  with  dilute  H2S04  (Fig. 
4,  p.  43). 

Chemical  Composition.     Calcium  carbonate,  CaCO3;  (CaO  = 


CARBONATES  295 

56.0  per  cent.) .  The  common  replacing  elements  are  iron,  magne- 
sium, and  manganese.  The  amber  color  is  due  to  a  small  amount 
of  organic  matter.  Clay,  sand,  bitumen,  and  other  mechanical 
impurities  may  be  present. 

Blowpipe  Tests.  Infusible,  glows,  and  gives  a  yellowish-red 
flame  coloration.  In  the  closed  tube  whitens,  gives  off  CO2,  and 
leaves  a  residue  of  CaO. 

Easily  soluble  in  large  fragments  in  cold  dilute  HC1  with  vigor- 
ous effervescence.  In  concentrated  solutions,  dilute  H2SO4  gives 
a  white  crystalline  precipitate  (CaS04-2H20). 

Distinguishing  Features.  Calcite  is  distinguished  from  dolo- 
mite by  its  lower  specific  gravity  and  by  its  ready  effervescence  in 
cold  HC1.  The  colored  varieties  are  distinguished  from  siderite 
and  rhodocrosite  by  lower  specific  gravity.  From  aragonite  it  is 
distinguished  by  perfect  rhombohedral  cleavage  and  by  its  failure 
to  give  a  lilac  color  when  heated  in  a  test-tube  with  cobalt  nitrate 
solution. 

Uses.  Limestones  are  extensively  used  for  building  and  orna- 
mental stones,  in  the  manufacture  of  cement,  as  ballast  and  road 
material  and  as  a  flux  in  smelting.  Iceland  spar  is  used  in  optical 
apparatus,  especially  the  polarizing  microscope. 

Occurrence.  1.  As  a  vein  mineral,  often  forming  the  gangue 
of  ores.  The  north  of  England  furnishes  fine  crystallized  speci- 
mens of  calcite. 

2.  As  travertine,  calcareous  tufa,  and  cave-deposits  (stalactites 
and  stalagmites).     Calcium  carbonate  is  soluble  in  carbonated 
water,  and  on  the  escape  of  CO2,  due  to  release  of  pressure,  calcite 
crystallizes  out. 

3.  As  a  biogenic  mineral  forming  limestones,  organisms  such 
as  molluscs,  brachiopods,  corals,  and  crinoids  contributing  their 
shells  or  other  hard  parts. 

4.  As  a  characteristic  mineral  in  cavities  of  the  basic  igneous 
rocks,  especially  basalt,  and  often  associated  with  the  zeolites. 
Iceland  spar  occurs  in  large  cavities  in  basalt  in  Iceland. 

5.  As  a  prominent  mineral  in  seams  and  cavities  of  sedimentary 


296        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

rocks,  especially  limestone.  In  the  Joplin  district  fine  amber- 
colored  calcite  crystals  occur  in  chert  breccias  in  the  zinc  mines 
along  underground  water  courses.  Crystal  Cave  at  Joplin  is 
completely  lined  with  calcite  crystals  from  1  to  2  feet  in  length. 
It  is  probable  that  the  calcite  was  formed  when  the  cave  was 
completely  filled  with  a  water  solution  of  calcium  carbonate. 

6.  As  the  principal  constituent  of  crystalline  limestones,  which 
were  formed  from  sedimentary  limestones  by  metamorphism. 
The   crystalline  limestones   often   contain   diopside,   tremolite, 
wollastonite,  garnet,  spinel,  graphite,  etc. 

7.  As  a  paramorph   after  aragonite.     Calcite  is  the   stable 
form  of  calcium  carbonate  under  ordinary  conditions. 

DOLOMITE,  Ca(Mg,Fe)(CO3)2 

Form.     Dolomite  is  found  in  crystals,  in  crystal  druses,  in 
cleavable  masses,  and  in  granular  and  massive  forms. 


FIG.  463. 


FIG.  464. 


FIG.  465. 


FIG.  466. 


The  crystals  belong  to  the  rhombohedral  class  of  the  hexagonal 
system  and  have  a  lower  grade  of  symmetry  than  calcite  crystals, 
though  this  is  not  often  apparent  on  inspection.  The  only  com- 
mon kind  of  dolomite  crystal  is  the  simple  unit  rhombohedron, 
often  curved  and  more  or  less  saddle-shaped  (Fig.  463) .  Figure 
464  is  a  rhombohedron  modified  by  c{0001[  and  Af{4041}. 
Crystals  like  Fig.  465  wth  cjOQOl  j  and  M"{4041}  are  found  em- 
bedded in  anhydrite  and  gypsum. 

Several  twinning  laws  are  known  for  dolomite,  but  the  only 
common  one  is  polysynthetic  twinning  with  {0221}  as  twin-plane 
which  gives  rise  to  striations  on  the  cleavage  faces  parallel  to 


CARBONATES  297 

both  the  short  diagonal  and  the  long  diagonal  of  the  rhomb  (Fig. 
466) .  This  test  can  often  be  used  to  distinguish  dolomite  from 
calcite  in  crystalline  limestones. 

Cleavage,  rhombohedral  like  that  of  calcite,  but  often  curved. 

H.  =  3J^  to  4.  Sp.  gr.  =  2.83  -  3.00  (varies  with  iron 
content) . 

Color,  white,  pink  or  gray,  but  rarely  colorless.  Luster,  pearly 
or  vitreous.  It  is  often  called  pearl  spar. 

Optical  Properties,  n/1.682)  -  na(1.503)  =  0.179.  Double 
refraction  very  strong.  Fragments  are  rhombic  with  symmetri- 
cal extinction  and  very  high-order  interference  colors.  The 
relief  varies  with  the  direction  as  in  calcite.  It  gives  the  micro- 
chemical  gypsum  test  with  dilute  H2SO4.  In  fragments  it  is 
distinguished  from  calcite  by  the  absence  of  striations  parallel  to 
the  long  diagonal. 

Chemical  Composition.  Calcium  magnesium  carbonate,  Ca- 
Mg(CO3)2;  (For  normal  dolomite  CaO  =  30.4  per  cent.;  Mg  = 
21.7  per  cent.  MgCO3  =  45.6).  It  often  contains  iron.  The 
highly  ferriferrous  variety  is  known  as  ankerite.  Ferriferous 
varieties  are  represented  by  the  formula  Ca(Mg,Fe)(CO3)2. 
Analyses  are  given  on  page  291. 

Dolomite  is  a  double  salt  with  equal  molecular  quantities  of 
CaCO3  and  MgC03,  and  not  an  isomorphous  mixture  of  these 
two  compounds. 

Blowpipe  Tests.     Infusible  and  colors  the  flame  yellowish  red. 

The  iron-bearing  varieties  darken  in  the  closed  tube  and  also 
become  magnetic  when  heated  in  R.F.  on  charcoal. 

Large  fragments  are  only  slightly  attacked  by  cold  dilute 
HC1.  (Dolomite  can  thus  be  distinguished  from  calcite.)  In  con- 
centrated solutions  dilute  H2S04  gives  a  white  crystalline  pre- 
cipitate. 

Distinguishing  Features.  The  curved  unit  rhombohedral 
crystals  and  pearly  luster  usually  distinguish  dolomite  from 
calcite.  Rhodochrosite  is  heavier  than  pink  dolomite. 

Uses.     Dolomitic   limestones,   like   ordinary    limestones,    are 


298        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

used  for  building  and  ornamental  purposes,  and  also  for  furnace 
linings. 

Occurrence.  1.  As  a  characteristic  mineral  in  cavities  of  lime- 
stones. The  pink  dolomite  of  the  Joplin  district  is  a  prominent 
example  of  this  occurrence. 

2.  As  the  essential  constituent  of  dolomitic  or  magnesian  lime- 
stones.    These  limestones  are  formed  from  ordinary  limestones 
by  the  process  known  as  dolomitization.     This  consists  of  the 
partial  replacement  of  calcium  carbonate  by  magnesium  carbon- 
ate in  a  way  not  fully  understood.     As  there  is  a  shrinkage  of 
about  10  per  cent.,  these  dolomitic  limestones  are  often  porous. 

3.  As  a  vein  mineral  often  associated  with  calcite,  as  in  the 
north  of  England. 

4.  As  the  principal  constituent  of  the  crystalline    dolomitic 
limestones.     These  consist  either  of  dolomite  or  of  a  mixture  of 
dolomite  and  calcite.     Other  characteristic  minerals  are  tremo- 
lite,    phlogopite,  chondrodite,  forsterite,  spinel,  antigorite,  and 
talc.     Antigorite  and  talc  are  secondary  minerals  formed  from 
the  other  silicates. 

Magnesite,  MgCO3 

Form.  Magnesite  occurs  in  cleavable  or  compact  porcelain- 
like  masses.  The  so-called  amorphous  magnesite  is  microcrys- 
talline  (metacolloid)  and  sometimes  has  a  colloform  surface. 
Euhedral  crystals  of  magnesite  are  exceedingly  rare. 

Cleavage.     Rhombohedral  cleavage  is  sometimes  prominent. 

Color,  usually  white  or  gray.     Luster,  vitreous  to  dull. 

H.  =  4  to5j£.  Sp.  gr.  3.1  ±. 

Optical  Properties.  n7(1.717)  •-  ntt(1.515)  =  0.202.  Frag- 
ments are  rhombic  with  symmetrical  extinction,  or  irregular  with 
aggregate  structure.  The  interference  colors  are  very  high- 
order. 

Chemical  Composition.  Magnesium  carbonate,  MgCOs ;  (MgO 
=  47.6  per  cent.).  Iron  and  calcium  are  often  present  in  small 
amounts.  The  massive  varieties  often  contain  magnesium 
silicate. 


CARBONATES  299 

Blowpipe  Tests.  Infusible.,  Turns  pink  when  heated  with 
cobalt  nitrate  solution. 

Soluble  in  hot  HC1  with  effervescence. 

Distinguishing  Features.  Magnesite  is  usually  distinguished 
by  the  compact  white  masses,  but  some  varieties  show  good 
cleavage  and  are  distinguished  by  the  comparatively  high  specific 
gravity.  It  may  be  necessary  to  prove  that  calcium  is  not  an 
essential  constituent. 

Uses.  Dead-burned  magnesite  is  used  as  a  refractory  lining. 
The  light-burned  or  caustic  magnesite  is  used  as  a  plaster  and 
in  the  manufacture  of  wood  pulp.  Austria-Hungary  and  Greece 
are  the  principal  producers  of  magnesite. 

Occurrence.  1.  In  veins  in  serpentine  (compact  massive 
variety).  Numerous  localities  in  California. 

2.  In  beds  as  a  replacement  of  limestone  or  dolomite  (cleavable 
variety).  Stevens  county,  Washington. 

SJDERITE,  FeCO3 

Form.  Siderite  is  found  in  small  crystals  in  cavities,  in  cleav- 
able masses,  in  colloform  crusts,  and  in  compact  masses. 

The  crystals  are  varied  in  habit;  the  common 
forms  are  the  unit  rhombohedron  (1011  :  1101 
=  73°  2H')i  the  rhombohedron  {OlT2},  and 
the  rhombohedron  {0221},  the  latter  often 
modified  by  the  pinacoid  {0001}.  Figure  467 
is  the  unit  rhombohedron  rflOll}.  Crystals 
may  be  curved  like  those  of  dolomite. 

Cleavage,  rhombohedral,  like  that  of  calcite. 

H.  =  3^  to  4.  Sp.  gr.  3.8  ±. 

Color,  various  tints  and  shades  of  brown  and  gray. 

Optical  Properties.  rcT(1.87)  -  ntt(1.63)  =  0.24.  Strong 
double  refraction.  Fragments  are  rhombic  with  symmetrical 
extinction  and  very  high-order  interference  colors.  In  methylene 
iodid  the  relief  changes  with  the  direction,  but  in  both  positions 
the  index  of  refraction  is  greater  than  that  of  methylene  iodid. 


300        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Chemical  Composition.  Ferrous  carbonate,  FeCO3;  (FeO  = 
62.1  per  cent.).  Calcium,  magnesium,  and  manganese  are 
usually  present  in  small  amounts  as  replacing  elements.  Clay- 
ironstone  is  an  impure  massive  siderite  containing  argillaceous 
material. 

Blowpipe  Tests.  Fuses  with  difficulty.  In  the  closed  tube 
it  darkens.  Heated  on  charcoal  in  R.F.  it  becomes  magnetic. 

It  is  soluble  in  hot  HC1  with  effervescence  and  the  solution 
gives  tests  for  ferrous  iron. 

Distinguishing  Features.  Siderite  is  distinguished  from  brown 
calcite  by  its  comparatively  high  specific  gravity,  and  from 
sphalerite  by  its  rhombohedral  cleavage  and  vitreous  luster. 

Uses.  Siderite  is  one  of  the  minor  ores  of  iron.  The  clay- 
ironstone  variety  has  been  mined  extensively  in  England  and  to 
some  extent  in  Ohio,  Pennsylvania,  and  Maryland. 

Occurrence.  1.  As  a  vein  mineral,  often  the  gangue  of  other 
ores. 

2.  As  a  metasomatic  replacement  of  limestone. 

3.  As  "  clay-ironstone "  concretions  in  shales  and  as  "  black- 
band"  ore,  a  carbonaceous  variety. 

4.  As  a  secondary  mineral  in  cavities  of  basalt. 

Rhodochrosite,  MnCO3 

Form.     Rhodochrosite  occurs  in  rhombohedral  crystals  and  in 
cleavable  masses.     The  unit  rhombohedron  with  (1011  :  1101)  = 
73°  0'  is  the  only  common  kind  of  crystal  (Fig. 
468). 

Cleavage,  rhombohedral,  like  that  of  calcite. 
H.  =  4.  Sp.  gr.  3.5  ±. 

Color,  pink,  red,  brownish-red  or  reddish- 
gray. 

Optical  Properties.  nT(1.82)  -  na(1.60)  = 
0.22.  Double  refraction  strong.  Fragments  are  rhombic  with 
symmetrical  extinction  and  have  very  high-order  interference 
colors. 


CARBONATES  301 

Chemical  Composition.  Manganous  carbonate,  MnCO3; 
(MnO  =  61.7  per  cent.).  Calcium  and  iron  are  usually  present 
as  replacing  elements. 

Blowpipe  Tests.  Infusible.  In  the  closed  tube  it  darkens 
and  decrepitates.  The  borax  bead  in  O.F.  is  amethyst  color. 

Soluble  in  hot  HC1  with  effervescence. 

Distinguishing  Features.  Rhodochrosite  is  distinguished  from 
pink  calcite  and  dolomite  by  its  higher  specific  gravity.  It 
even  more  resembles  rhodonite  (MnSiOs)  but  is  distinguished  by 
its  rhombohedral  cleavage  and  inferior  hardness. 

Uses.     Rhodochrosite  is  sometimes  an  ore  of  manganese. 

Occurrence.  1.  A  characteristic  vein  mineral  often  serving  as 
the  gangue  of  silver  ores.  Lake  county,  Colorado. 

SMITHSONITE,  ZnCO3 

Form.  Smithsonite  usually  occurs  in  colloform  incrustations 
and  in  porous  masses.  Crystals  of  smithsonite  are  comparatively 
rare  and  as  a  rule  are  much  rounded. 

Cleavage,  imperfect  rhombohedral  and  often  curved. 

H.  =  5.  Sp.  gr.  4.4  ±. 

Color,  white,  gray,  yellow,  brown;  sometimes  blue  or  green. 

Optical  Properties.  ny(l.S2)  -na(1.62)  =0.20.  Strongdouble 
refraction.  Fragments  are  rhombic  with  symmetrical  extinction 
and  very  high-order  interference  colors.  The  index  of  refraction 
is  greater  than  that  of  methylene  iodid. 

Chemical  Composition.  Zinc  carbonate,  ZnCO3;  (ZnO  = 
64.8  per  cent.;  Zn  =  52.1  per  cent.).  Iron  is  the  most  frequent 
replacing  element. 

Blowpipe  Tests.  Infusible.  Heated  with  cobalt  nitrate  solu- 
tion on  charcoal,  it  gives  a  green  coating.  In  the  closed  tube 
it  turns  yellow. 

Soluble  in  HC1  with  effervescence. 

Distinguishing  Features.  It  resembles  calamine  but  is 
distinguished  by  the  absence  of  crystals  with  sharp  edges.  It  is 
harder  than  the  other  carbonates. 


302        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Uses.  Smithsonite  is  one  of  the  minor  ores  of  zinc  and  is  often 
associated  with  calamine,  a  basic  zinc  silicate.  It  is  called  "  dry- 
bone"  in  the  Wisconsin-Illinois-Iowa  zinc  district. 

Occurrence.  1.  Usually  found  in  the  oxidized  zone  and 
formed  from  sphalerite.  It  also  occurs  as  a  metasomatic  replace- 
ment of  limestone,  and  is  often  pseudomorphous  after  calcite 
and  dolomite.  Marion  county,  Arkansas,  is  a  prominent 
locality. 

ARAGONITE  GROUP— ORTHORHOMBIC 

The  aragonite  group  is  an  isomorphous  group  consisting  of 
aragonite  CaCO3;  strontianite,  SrC03;  witherite,  BaCO3;  and 
cerussite,  PbCO3.  The  crystals  are  orthorhombic,  but  pseudo- 
hexagonal  (110  :  110  =  62°  —  64°),  and  are  usually  prismatic 
in  habit.  Twinning  on  the  unit  prism  { 110}  is  very  common  and 
also  accounts  for  the  pseudohexagonal  character  of  some  crystals. 
Optically  the  minerals  are  biaxial  with  a  small  axial  angle.  The 
double  refraction  is  very  strong  as  in  the  calcite  group. 

Aragonite,   CaCO3 

Form.  Aragonite  occurs  in  acicular  crystals,  in  columnar 
and  fibrous  masses,  and  in  incrusting  and  stalactitic  forms. 
Fibrous  masses  are  especially  common  for  aragonite. 

The  crystals  are  usually  prismatic  or  acicular  in  habit,  and 
belong  to  the  bipyramidal  class  of  the  orthorhombic  system. 
Usual  forms:  mjllO),  6  {010},  ft  {Oil}.  Interfacial  angles: 
mm(110  :  HO)  =  63°  48';  w&(110  :  010)  =  58°  6';  &fc(010  : 
Oil)  =  54°  13K'.  Figures  469  and  470  are  drawings  of  typical 
crystals.  The  pseudohexagonal  character  can  be  seen  from 
Figure  471.  Twins  with  raj  110}  as  twin-plane  are  common. 
Figure  472  is  a  contact  twin  and  Fig.  473,  a  penetration-trilling 
with  slight  reentrant  angles. 

Cleavage,  imperfect  parallel  to  the  length  of  the  crystals  (010 
face).  Calcite  has  perfect  cleavage  oblique  to  the  length  of  the 
crystals. 

H.  =  3J£.  Sp.  gr.    2.9  ±. 


CARBONATES 


303 


Color,  colorless,  white,  or  amber;  also  other  pale  tints.  Luster, 
vitreous;  faint  resinous  on  fracture. 

Optical  Properties.  ny  =  (1.68)  -  na  (1.53)  =  0.15.  Double 
refraction  very  strong.  Fragments  are  prismatic  with  parallel 
extinction,  negative  elongation,  and  very  high-order  interference 
colors.  With  dilute  H2SO4,  the  hydrochloric  acid  solution  gives 
the  microchemical  gypsum  test. 

Chemical  Composition.  Calcium  carbonate,  CaCO3;  (CaO  = 
56.0  per  cent.).  It  often  contains  a  small  amount  of  SrCO3. 

Blowpipe  Tests.  Infusible,  but  becomes  opaque  and  falls  to 
pieces  when  heated. 


m 


m 


FIG.  469. 


FIG.  470.       FIG.  471.       FIG.  472. 
FIGS.  469-473. — Aragonite  crystals. 


FIG.  473. 


Soluble  in  cold  dilute  acids  with  effervescence.  In  a  concen- 
trated solution,  dilute  H2SO4  gives  a  crystalline  precipitate  of 
CaSO4-2H2O.  Finely  powdered  aragonite  heated  in  a  test-tube 
with  cobalt  nitrate  solution  becomes  lilac  colored,  while  calcite  is 
practically  unchanged. 

Distinguishing  Features.  Aragonite  and  calcite  are  dis- 
tinguished by  differences  in  crystal  form,  cleavage,  and  specific 
gravity  and  if  these  fail,  by  the  cobalt  nitrate  test. 

Occurrence.  1.  As  a  secondary  mineral  in  basic  igneous 
rocks  such  as  basalt. 

2.  As  a  secondary  mineral  in  seams  and  cavities  of  limestones 
along  with  calcite. 


304        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

3.  In  clays  and  marls  along  with  gypsum.     Prominent  locali- 
ties are  Girgenti  in  Sicily  and  the  Pyrenees. 

4.  As  a  hot-spring  deposit.     Carlsbad,  Bohemia.     This  recalls 
the  fact  that  aragonite  often  forms  as  a  crust  in  a  tea-kettle. 

5.  As  a  mother-of-pearl  layer  of  mollusc  shells. 

6.  As  a  secondary  mineral  in  serpentine,  produced  by  the 
decomposition  of  the  pyroxene  of  the  original  peridotite. 

Strontianite,  SrCO3 

Form.  Strontianite  occurs  in  acicular  crystals  and  also  in 
columnar  and  fibrous  masses.  The  crystals  are  pseudohexag- 
onal  orthorhombic  and  resemble  those  of  aragonite. 

H.  =  3J£  to  4.  Sp.  gr.  3.7  ±. 

Color,  colorless,  white,  or  pale  tints. 

Optical  Properties.  7^(1.67)  -  na(1.52)  =  0.15.  Like  those 
of  aragonite,  but  gives  faint  test,  if  any,  for  microchemical 
gypsum. 

Chemical  Composition.  Strontium  carbonate,  SrCO3;  (SrO  = 
70.1  per  cent.).  It  usually  contains  some  CaCO3,  which  may  be 
detected  by  the  microchemical  gypsum  test. 

Blowpipe  Tests.  Infusible,  but  swells  up  and  gives  a  crimson 
flame  when  heated  after  it  is  moistened  with  HCl. 

Soluble  in  HCl  with  effervescence.  In  dilute  solutions, 
H2SO4  gives  a  finely  divided  white  precipitate  which  distinguishes 
Strontianite  from  aragonite. 

Distinguishing  Features.  The  fibrous  or  columnar  structure 
and  high  specific  gravity  are  characteristic.  It  may  be  mistaken 
for  celestite,  which  has  about  the  same  specific  gravity,  but 
the  solubility  in  HCl  will  differentiate  them. 

Uses.  The  strontium  hydroxid  used  in  sugar  refining  is  made 
from  Strontianite. 

Occurrence.  1.  As  a  secondary  mineral  produced  from 
celestite. 

2.  In  veins  in  calcareous  marl.  Hamm  in  Westphalia,  Ger- 
many, is  a  prominent  locality. 


CARBONATES  305 

Witherite,  BaCO3 

Form.  Witherite  occurs  in  granular  or  columnar  masses  in 
crystalline  druses,  and  in  distinct  crystals.  The  crystals  are 
pseudohexagonal  twins  of  pyramidal  habit,  often  resembling 
quartz  crystals.  (See  Fig.  474.-) 

H.  =  3K.  Sp.  gr.  4.3  ±. 

Color,    white   or   gray.     Luster,    faint   resinous   or   vitreous. 

Optical  Properties.  ny(1.67)  -  n«(1.52)  = 
0.15.  The  optical  properties  are  like  those  of 
aragonite. 

Chemical  Composition.  Barium  carbonate, 
BaCO3;  (BaO  =  77.7  per  cent.). 

Blowpipe  Tests.  Easily  fusible,  giving  a 
yellowish-green  flame. 

Soluble  in  HC1  with  effervescence.  In 
dilute  solution,  H2S04  gives  a  finely  divided 
white  precipitate. 

Distinguishing  Features.  The  high  specific  gravity  and  effer- 
vescence in  acids  distinguish  witherite  from  other  minerals. 

Occurrence.  1.  In  veins  with  galena.  Barite  is  a  secondary 
mineral.  The  North  of  England  is  the  only  prominent  locality 
for  witherite. 

CERUSSITE,  PbCO3 

Form.  Cerussite  occurs  in  fibrous  and  reticulated  forms,  in 
compact  masses,  and  also  in  crystals  which  are  orthorhombic 
and  pseudo-hexagonal.  The  habit  is  usually  tabular  parallel  to 
the  side  pinacoid  6(010}  (Fig.  476),  or  pyramidal  with  pflll) 
and  i{  021 }  in  about  equal  development  (Fig.  475).  Other  promi- 
nent forms  are:m{110),  ajlOO},  r{  130},  fc{011J. 

Twinning  (ra{100[  as  twin-plane)  is  common  both  as  simple 
contact  twins  like  Fig.  477  and  as  interpenetrant  twins  like  Fig. 
478. 

H.  =  3^.  Sp.gr.  6.5±. 

20 


306        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Color,  colorless,  white,  gray,  and  other  tints.  Luster,  adaman- 
tine. Very  brittle. 

Optical  Properties.  n7(2.07)-na(1.80)  =  0.27.  Double  re- 
fraction very  strong.  Fragments  are  mostly  irregular  with  very 
high-order  interference  colors,  and  index  of  refraction  greater  than 
methylene  iodid.  A  nitric  acid  solution  gives  octahedral  crystals 
of  lead  nitrate. 

Chemical  Composition.  Lead  carbonate,  PbCOs,  PbO  = 
83.5  (Pb  =  77.5  per  cent.). 

Blowpipe  Tests.  Fuses  easily  on  charcoal.  In  the  closed  tube 
it  decrepitates  and  becomes  yellow.  Heated  on  charcoal  in 
R.F.,  cerussite  gives  a  metallic  button  and  a  yellow  coating  (PbO). 


FIG.  475. 


FIG.  476. 


FIG.  477. 


FIG.  478. 


Soluble  in  HN03  with  effervescence.  Soluble  in  hot  HC1,  but 
on  cooling  needle  crystals  of  adamantine  luster  (PbCl2)  separate. 

Distinguishing  Features.  Distinguished  from  most  other  min- 
erals by  adamantine  luster  and  high  specific  gravity.  It  may 
usually  be  distinguished  from  anglesite  by  the  presence  of  twin- 
ned crystals,  but  chemical  tests  may  be  necessary. 

Uses.  Cerussite  is  an  ore  of  lead  and  sometimes  is 
argentiferous. 

Occurrence.  1.  Usually  derived  from  galena  and  occurring 
especially  in  the  gossan  of  ore-deposits.  Prominent  localities 
are  Broken  Hill,  New  South  Wales,  and  Coeur  d'Alene  district, 
Idaho.  In  the  Joplin  district  cerussite  pseudomorphs  after 
galena  are  found. 


CARBONATES  307 

MALACHITE,  Cu2(OH)2CO3 

Form.  For  malachite  the  characteristic  recurrences  are  mam- 
millary  crusts,  fibrous  masses,  and  acicular  crystals.  The  crystals 
are  monoclinic,  but  are  usually  very  small  and  indistinct. 

H.  =  3K  to  4  '     Sp.  gr.  3.9  ±. 

Color,  emerald  green. 

Optical  Properties.  nT(1.91 )-  w«(1.66)  =  0.25.  Fragments 
are  prismatic  with  oblique  extinction  (23°).  Interference  colors 
are  masked  by  the  green  color  of  the  mineral.  Arrow-head  twins 
like  those  of  gypsum  (Fig.  238)  are  common  among  the  fragments. 

Chemical  Composition.  Basic  copper  carbonate,  Cu2(OH)CO3 
or  CuCO3-Cu(OH)2;  (Cu  =  57.4  per  cent.;  H20  =  8.1  per  cent.). 

Blowpipe  Tests.  Easily  fusible  at  (3)  giving  a  green  flame 
which  is  made  blue  by  HC1.  In  the  closed  tube  it  turns  black 
and  gives  off  water. 

Soluble  in  acids  with  effervescence  and  gives  a  green  solution. 

Distinguishing  Features.  It  is  distinguished  from  other  copper 
minerals  of  green  color  by  its  effervescence  in  acids.  Chryso- 
colla  is  blue-green  and  lacks  the  fibrous  structure  of  malachite. 

Uses.  Malachite  is  sometimes  an  ore  of  copper  together  with 
azurite,  cuprite,  and  chrysocolla,  which  are  collectively  called 
oxidized  ores.  Polished  malachite  with  a  concentric  fibrous 
structure  is  used  as  an  ornamental  stone. 

Occurrence.  1.  A  characteristic  mineral  of  the  upper  oxi- 
dized zone  of  copper  deposits.  It  constitutes  the  so-called  "  cop- 
per-stain, "  found  in  outcrops.  Malachite  is  often  associated  with 
azurite  and  is  sometimes  pseudomorphous  after  it.  Bisbee, 
Arizona,  is  a  prominent  locality. 

Azurite,  Cu3(OH)2(CO3)2 

Form.  Azurite  occurs  in  crystals  in  crystalline  coatings  and 
nodular  groups  of  crystals.  Crystals  are  monoclinic,  prismatic 
class,  and  are  usually  short  prismatic  or  tabular  in  habit,  often 


308        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

highly  modified.  The  best  specimens  come  from  Chessy  in 
France,  hence  chessylite,  the  French  name  for  azurite. 

H.  =  4.  Sp.  gr.  3.8±. 

Color,  deep  azure  blue. 

Optical  Properties.  ri>1.83.  Fragments  are  irregular,  blue 
in  color,  but  not  pleochroic.  Interference  colors  are  masked. 

Chemical  Composition.  Basic  copper  carbonate,  Cu3(OH)2- 
C03)2  or  2CuC03-Cu(OH)2;  (Cu  =  55.2  per  cent.,  H2O  =  5.2 
percent.). 

Blowpipe  Tests.     The  same  as  for  malachite. 

Distinguishing  Features.  The  dark  azure  blue  color  is  dis- 
tinctive. It  differs  from  malachite  not  only  in  color,  but  also  in 
water  content. 

Uses.     Azurite  is  one  of  the  so-called  oxidized  copper  ores. 

Occurrence.  A  characteristic  mineral  of  the  oxidized  zone. 
It  is  usually  associated  with  malachite  and  has  been  formed 
after  the  malachite.  Bisbee,  Arizona,  has  furnished  fine  speci- 
mens of  azurite. 

Hydromagnesite,  Mg4(OH)2(CO3) ,  3H2O 

Form.  Hydromagnesite  occurs  in  crystalline  crusts,  thin 
seams,  and  earthy,  chalk-like  masses.  Crystals,  which  are  very 
small,  are  usually  like  Fig.  479  and  are  monoclinic  and  usually 
twinned  on  {100}. 

H.  =  1  to  3.  Sp.  gr.  2.1±. 

Color,  white,.    Luster,  often  pearly. 

Optical  Properties.  n7(1.545)  -  n«(1.523)  =  0.022.  Frag- 
ments and  crystals  are  prismatic  with  parallel  extinction,  nega- 
tive elongation,  and  low-order  interference  colors. 

Chemical  Composition.  Hydrous  basic  magnesium  carbonate, 
Mg4(OH)2(C03)3-3H2O  or  3MgCO3-Mg(OH)2  3H2O;  (H20  = 
19.8  per  cent.). 

Blowpipe  Tests.     Infusible.     In  the  closed  tube  it  gives  water. 

Soluble  in  HC1  with  effervescence. 


CARBONATES  309 

Distinguishing  Features.  .Hydromagnesite  is  distinguished 
from  magnesite  by  its  inferior  hardness  and  abundant  water. 

Occurrence.  1.  As  a  secondary  mineral  in  serpentine.  Ala- 
meda  county,  California  (Fig.  479). 


FIG.  479. 


2.  In   crystalline   limestones,  formed   directly   from    brucite, 
which  in  turn  was  formed  from  periclase  (MgO).     Crestmore, 
Riverside  county,  California. 

3.  In  spring  deposits.     Atlin,  British  Columbia. 


9.  PHOSPHATES,  NITRATES,  BORATES,  ETC. 


Apatite  Group 


Ilmenite, 

Columbite, 

APATITE, 

Dahllite, 

Pyromorphite, 

Mimetite, 

Vanadinite, 

COLLOPHANE, 

Turquois, 

Carnotite, 

Nitratine, 

Colemanite, 

Ulexite, 

Pitchblende, 


FeTi03 

(Fe,Mn)(Nb,Ta)2O6 

Cai0F2(PO4)6 

Ca10(C03)(P04)6 

Pb10ClHP04)6 

PbioCl2(As04)6 

Pb10Cl2(V04)6 

8Cas(P04),-Ca(CO*F,)(H,0), 

H5[Al(OH)2]6-Cu(OHXP04)4 

K2(U02)2(V04)2-8H20 

NaN03 

Ca2B6On-6H2O 

NaCaB5O98H2O 

(UO,),U04-(H,0), 


About  150  phosphate  minerals  are  known  but  most  of  them  are 
rare.  Basic  phosphates  of  iron  and  of  copper  are  especially 
numerous.  The  compounds  are  practically  all  orthophosphates, 
that  is,  salts  of  H3PO4. 

Ilmenite  and  columbite  properly  belong  to  other  divisions,  but 
are  placed  here  for  convenience. 

Of  the  few  mineral  nitrates  known,  nitratine  (sodium  nitrate) 
is  the  only  one  of  importance.  Only  a  few  borates  are  prominent 
as  minerals.  H3BO3  is  boric  acid.  Borax  is  a  salt  of  H2B4O7, 
which  is  derived  thus :  4H  3BO 3  —  5H20  =  H2B4O7.  Colemanite  is 
a  salt  of  H4B6On  (6H3B03-  7H2O).  These  are  called  tetra-  and 
hexa-boric  acids  respectively. 

Ilmenite,  FeTiO3 

Form.  Ilmenite  occurs  in  tabular  hexagonal  crystals,  in  flat 
plates  without  definite  outline,  in  disseminated  grains,  in  com- 

310 


PHOSPHATES,  NITRATES,  BORATES,  ETC. 


311 


pact  masses,  and  in  the  form  of  sand.  The  crystals  resemble 
those  of  hematite  in  habit  and  angles,  but  the  crystal  class  is 
different,  for  ilmenite  belongs  to  the  rhombohedral  class. 

H.  =  5  to  6.  Sp.gr.  4.7  + . 

Color,  black.  Luster,  submetallic  to  metallic.  Streak,  black 
to  brownish-red.  Slightly  magnetic. 

Chemical  Composition.  Ferrous  metatitanate,  FeTiO3;  (FeO 
=  47.3  per  cent.),  analogous  to  a  metasilicate.  Ilmenite  usually 
contains  ferric  iron  and  also  magnesium.  It  grades  on  the  one 
hand  into  hematite  and  on  the  other  into  MgTi03  (geikielite) . 

Analyses  of  the  Minerals  of  the  Ilmenite  Group 


FeO 

MgO 

MnO 

TiO2 

Fe203 

Ilmenite  ...      .                

22.4 

0.5 

0.3 

23.7 

53.7 

Ilmenite                                                    .  .  . 

36.5 

0.6 

2.7 

45.9 

14.3 

Ilmenite  (Picroilmenite)  

24.4 

14.2 

56.1 

5.4 

Geikielite                                       

6.3 

28.5 

63.8 

1.9 

Blowpipe  Tests.  Infusible.  The  sodium  carbonate  fusion 
dissolved  in  HC1  and  boiled  with  metallic  tin  gives  a  violet 
solution. 

Slowly  soluble  in  HC1.     Decomposed  by  fusion  with  KHS04. 

Distinguishing  Features.  Ilmenite  may  be  mistaken  for 
hematite  or  magnetite.  It  fails  to  give  the  red  streak  of  the 
former  and  the  strong  magnetism  of  the  latter.  When  it  is 
intimately  mixed  with  hematite  and  magnetite,  polished  surfaces 
are  necessary  to  identify  it. 

Occurrence.  1.  As  an  accessory  constituent  of  igneous  rocks, 
especially  diabases. 

2.  As  a  magmatic  segregation  in  igneous  rocks  intergrown 
with  magnetite  and  forming  the  so-called  titaniferous  magnetites. 

3.  As  a  prominent  constituent  of  sands,  especially  the  "  black 
sands." 


312        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Columbite,  (Fe,Mn)  (Nb,Ta)  2O  6 

Form.  Columbite  usually  occurs  in  orthorhombic  crystals  of 
short  prismatic  or  tabular  habit. 

Cleavage,  fair  in  two  directions  at  right  angles. 

H.  =  6.  Sp.  gr.  5.5-6.5. 

Color,  black,  often  iridescent.    Luster,  submetallic. 

Chemical  Composition.  An  iron  and  manganese  meta-niobate 
and  meta-tantalate  grading  from  (Fe,Mn)Nb2O6  to  (Fe,Mn) 
Ta206.  The  latter  mineral  is  called  tantalite. 

Blowpipe  Tests.  Fusible  on  the  edges  with  difficulty  (5^). 
On  charcoal  with  sodium  carbonate  in  R.F.  it  gives  a  magnetic 
residue.  The  sodium  carbonate  bead  in  O.F.  is  opaque  bluish- 
green  (Mn.).  The  borax  fusion  dissolved  in  HC1  with  tin  gives 
a  deep  blue  color. 

Distinguishing  Features.  Columbite  resembles  wolframite 
and  gives  almost  identical  blowpipe  tests,  but  the  latter  has 
higher  specific  gravity  and  more  perfect  cleavage. 

Insoluble  in  acids. 

Uses.  Tantalum,  which  is  used  as  a  filament  for  incandescent 
lights,  is  obtained  from  columbite  and  tantalite. 

Occurrence.  1.  In  granite  pegmatites,  associated  with  beryl, 
lepidolite,  spodumene,  etc.  The  Etta  mine  in  the  Black  Hills, 
South  Dakota,  is  the  most  prominent  locality  in  the  United 
States. 

APATITE   GROUP— HEXAGONAL 

The  apatite  group  is  a  well-established  isomorphous  group  of 
minerals,  for  several  isomorphous  mixtures  are  known  as  given  in 
the  analyses  below.  Besides  fluor-apatite  3Ca3(PO4)2-CaF2  and 
chlor-apatite,  3Ca3(PO4)2-CaCl2,  there  are  dahllite,  3Ca3(PO4)2- 
CaCO3,  voelckerite,  3Ca3(PO4)2-CaO,  and  a  rare  mineral  called 
svabite,  3Ca(AsO4)2-CaF2.  The  following  are  typical  analyses  of 
the  minerals  of  this  group  (given  in  the  form  of  metals  and  acid 
radicals  instead  of  the  usual  form). 


PHOSPHATES,  NITRATES,  BORATES,  ETC. 
Analyses  of  Minerals  of  the  Apatite  Group 


313 


Ca 

Pb 

P04 

AsO4 

V04 

Cl 

F 

Misc. 

Fluor-apatite,  Portland 

39  5 

55  4 

0  ?, 

3  8 

1  .1 

Chlor-apatite,  Norway 

37  2 

54  2 

R  0 

3.1 

Voelckerite,  Zillerthal 

40  4 

57  5 

0  6 

O  =  1.4, 

Dahllite  (Podolite)    Russia 

36  5 

52  2 

H2O  =  0.1 
COs  =  53, 

Pyromorphite,  Freiberg 

4  6 

67  0 

26  0 

0  8 

1  9 

Fe2Os  =  3.0 

Pyromorphite,  Schemnitz 

0  2 

75   1 

21  3 

2  5 

Pyromorphite,  Roughten  Gill.  .  . 
Mimetite,  Bohemia  
Endlichite,  New  Mexico  
Vanadinite,  Arizona  
Svabite,  Jakobsberg  

0.2 
30.1 

71.7 
70.6 
68.2 
71.7 

2.8 

15.1 
0.8 
tr 
1.9 
0.5 

10.9 
26.7 
16.1 

61.7 

13.7 

24.7 

2.3 
2.5 
2.5 
2.4 
0.1 

2.0 

1.7 
2.6 

APATITE,  CaioF2(PO4)6 

Form.     Apatite  is  found  in  crystals    and  in  massive  forms. 
The  crystals  are  hexagonal  and  belong  to  the  bipyramidal  class 


FIG.  480. 


FIG.  481. 


FIG.  482. 


FIG.  483. 


as  there  is  but  one  plane  of  symmetry  which  is  horizontal.  The 
habit  is  usually  prismatic  with  the  following  forms:  mjlOlO}, 
c|0001[,  andzjlOll}.  Interfacial  angles:  raz(10TO:10Tl)  =  49° 
42';  aa(10Tl:OlTl)  =  37°  44^'-  Figures  480  to  483  represent 
usual  types  of  crystals.  In  Fig.  482,  the  general  form  ^{2131}  is 
present  in  addition  to  s{  1121}  and  other  forms. 

Cleavage,  imperfect  basal  parallel  to  {0001}. 

H.  =  5.  Sp.  gr.  3.2  ±. 


314        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Color,  usually  reddish-brown  or  green,  more  rarely  white, 
colorless,  gray,  or  violet.  Luster,  subresinous. 

Optical  Properties.  w7(  1.646)  -  na(1.641)  =  0.005.  Frag- 
ments are  irregular,  and  colorless  with  low  first-order  interference 
colors.  It  gives  the  microchemical  gypsum  test  with  dilute 
H2SO4. 

Chemical  Composition.  Apatite  is  an  isomorphous  mixture  of 
the  tricalcium  fluophosphate,  CaioF2(PO4)6  with  smaller  amounts 
of  CaioCl2(P04)6,  CaioO(TO4)6,  and  Caio(C03)(PO4)6.  For 
CaioF2(TO4)6  (Ca  =  39.7  per  cent.,  F  =  3.8  per  cent.,  P04  = 
56.5  per  cent.).  Some  varieties  contain  manganese. 

Blowpipe  Tests.  Fusible  on  edges  (5M)  and  gives  a  yellowish- 
red  flame  which  is  made  green  with  H2SO4. 

Soluble  in  HNO3,  sometimes  with  slight  effervescence  on  heat- 
ing. The  solution  gives  a  yellow  precipitate  with  an  excess  of 
(NH4)2Mo04  solution  when  warmed.  As  NH4OH  gives  a  white 
precipitate  of  calcium  phosphate,  the  best  test  for  calcium  is 
dilute  H2S04  which  in  the  presence  of  50  per  cent,  alcohol  gives 
a  crystalline  precipitate  of  CaS04-2H2O. 

Distinguishing  Features.  Apatite  is  distinguished  from  most 
similar  minerals  by  its  crystal  form,  hardness  (just  less  than  a 
knife  blade),  and  subresinous  luster.  Collophane  is  amorphous, 
has  lower  specific  gravity,  and  effervesces  in  acids. 

Uses.  Massive  apatite  is  used  to  some  extent  in  the  prepara- 
tion of  superphosphate  for  fertilizer,  but  most  of  the  superphos- 
phate is  made  from  so-called  phosphate  rock  which  is  largely 
collophane. 

Occurrence.  1.  As  an  accessory  constituent  of  igneous  rocks, 
very  common  and  widely  distributed  but  in  small  quantities. 
It  is  the  original  source  of  the  phosphates  of  sedimentary 
rocks. 

2.  In  high-temperature    veins   with   phlogopite,   calcite,  and 
diopside. 

3.  In  metamorphic  rocks. 

4.  In  granite  pegmatites. 


PHOSPHATES,  NITRATES,  BORATES,  ETC.  315 

Dahllite,  Caio(CO3)(PO4)6 

Form.  Dahllite  occurs  in  colloform  fibrous  incrustations  on 
cellophane,  rarely  in  minute  hexagonal  crystals. 

H.  =  5.  Sp.  Gr.  =  3.0  ±. 

Color,  white  or  pale  tints. 

Optical  Properties.  n.,  (1.623)  -  wa(1.619)  =  .004.  Frag- 
ments are  prismatic  to  acicular  with  parallel  extinction,  nega- 
tive elongation,  and  low  to  middle  first  order  interference  colors. 

Chemical  Composition.  Calcium  carbonate-phosphate,  Cai0- 
(C03)(PO4)6,  analogous  to  fluor-apatite.  (Ca  =  38.9  per  cent. 
PO4  =  55.3,  CO 3  =  5.8.)  Water  is  usually  present,  but  it  is 
probably  mechanically  held  as  in  chalcedony. 

Blowpipe  Tests.  Fusible  with  difficulty,  but  usually  decrepi- 
tates. In  the  closed  tube  it  gives  a  small  amount  of  water. 

Soluble  in  cold  dilute  HNO3  with  slight  effervescence;  effer- 
vesces vigorously  in  hot  HN03.  The  solution  gives  tests  for 
calcium  with  dilute  H2S04  and  alcohol,  and  for  P04  with  ammo- 
nium molybdate. 

Distinguishing  Features.  Dahllite  is  distinguished  from 
cellophane  by  its  crystalline  character  and  small  water  content, 
and  from  apatite  by  its  effervescence  in  HN03. 

Uses.  Some  dahllite  occurs  in  phosphorites  (the  hard  rock 
phosphate  of  Florida,  for  example)  along  with  cellophane  and  is 
used  for  superphosphate. 

Occurrence.  1.  In  phosphorite  or  so-called  phosphate  rock 
as  an  incrustation  on  cellophane.  Marion  county,  Florida. 

Pyromorphite,  Pbi0Cl2(PO4)6 

Form.  Pyromorphite  usually  occurs  as  small  crystals  and 
earthy  crusts.  The  crystals  are  hexagonal  and  prismatic  in 
habit;  c{0001}  and  m{1010)  are  the  only  common  forms  (Fig. 
484). 

H.  =  3H  to  4.  Sp.  gr.  6.8 ±. 

Color,  green  or  brown.     Luster,  adamantine. 


316        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Optical  Properties.  ny  (2.061)  -na  (2.049)  =  0.012.  Frag- 
ments are  irregular,  colorless  or  pale  green,  and  have  low  first- 
order  interference  colors.  With  HNO3  isometric  crystals  of 
lead  nitrate  are  deposited. 

Chemical  Composition.  Lead  chlorid-phosphate,  PbioCl2- 
(P02)4;  (Pb  =  76.3  per  cent.).  In  some  varieties  Ca  replaces 
Pb  (p.  313),  and  in  others  (V04)  replaces  (PO4). 

Blowpipe  Tests.     Easily  fusible  (at  2)  on  charcoal  to  a  globule 
with  apparent  crystal  faces.     With  sodium  carbonate  on  char- 
coal  it  yields  a  lead  button.     With  a  NaPO3 
bead  saturated  with  CuO  it  gives  an  azure-blue 
flame  (Cl). 

Soluble  in  HNO3.     An  excess  of  (NH4)2  MoO4 
gives  a  yellow  ppt.  with  the  nitric  acid  solution. 
Distinguishing    Features      Pyromorphite    is 
distinguished  from  other  minerals  with  adaman- 
tine luster  by  its  hexagonal  form. 

Occurrence.  1.  As  a  secondary  mineral 
formed  from  galena.  Phoenixville,  Pennsylvania,  is  a  prominent 
locality.  Both  pyromorphite  pseudomorphs  after  galena  and 
galena  pseudomorphs  after  pyromorphite  have  been  noted,  the 
former  from  the  Joplin  district  and  the  latter  from  Huelgoat, 
France. 

Mimetite,  Pbi0Cl2(AsO4)6 

Form.     Mimetite  usually  occurs  in  rounded  hexagonal  crystals. 

H.  =  3M-  Sp.  gr.  7.2  ±. 

Color,  yellow,  orange,  or  red.     Luster  adamantine. 

Optical  Properties.  nT(2.13)  -  na(2.l2)  =  0.01.  Fragments 
are  irregular  yellow,  and  have  low  first-order  interference  colors. 
With  HNO3  isometric  crystals  of  Pb(NO3)2  are  deposited. 

Chemical  Composition.  Lead  chlorid-arsenate,  Pbi0Cl2(AsO4)  6; 
(Pb  =  69.5  per  cent.).  It  grades  on  the  one  hand  into  pyromor- 
phite and  on  the  other  into  vanadinite. 

Blowpipe  Tests.     Easily  fusible  (at  1^)  on  charcoal  and  gives 


FIG.  484. 


PHOSPHATES,  NITRATES,  BORATES,  ETC.  317 

a  white  sublimate  and  a  metallic  button.  In  the  closed  tube 
heated  with  charcoal  it  gives  an  arsenic  mirror  test  for  an  arsen- 
ate.  With  CuO  in  NaPO3  bead  it  gives  an  azure-blue  flame  (Cl). 

Soluble  in  HNO3.  With  (NH4)2MoO4  the  nitric  acid  solution 
gives  a  yellow  precipitate  on  boiling  (phosphates  give  the  pre- 
cipitate on  slight  warming) .  In  order  to  determine  the  presence 
of  the  phosphate  radical  it  is  necessary  to  remove  the  arsenic 
by  means  of  H2S. 

Distinguishing  Features.  Mimetite  is  difficult  to  distinguish 
at  sight.  The  adamantine  luster  and  high  specific  gravity 
suggest  that  it  is  a  lead  mineral. 

Occurrence.  1.  As  a  secondary  mineral  in  lead  mines.  Cum- 
berland, England. 

Vanadinite  Pb]0Cl2(VO4)6 

Form.  Vanadinite  practically  always  occurs  in  small  hexag- 
onal crystals  of  prismatic  habit.  The  common  forms  are :  c  { 000 1 } 

and_ra{10lOj     (Fig.    485).     The   general   form 

{2131},  a  hexagonal   bipyramid,  is  sometimes 
present. 

H.  =  3.  Sp.gr.  6.8  + . 

Color,  usually  red,  but  also  yellow  and  brown. 
Luster,  adamantine. 

Optical  Properties.  n7(2.35)-na(2.30)  - 
0.05.  Fragments  are  irregular  yellow  or  orange 
color,  and  give  bright  interference  colors. 

Chemical    Composition.      Lead    chlorid-van- 
adate,  PbioCl2(VO4)6;  (Pb  =  72.7  per  cent.).     It  often  contains 
(PO4)  and  (As04)  as  replacing  radicals.     Endlichite  is  intermedi- 
ate between  vanadinite  and  mimetite  (see  analyses,  page  313). 

Blowpipe  Tests.  Easily  fusible  (at  1^)  on  charcoal,  giving  a 
white  sublimate  and  a  metallic  globule.  In  the  closed  tube  with 
KHSO4  it  gives  a  yellow  mass.  The  NaPO3  bead  is  green  in 
R.F.  and  yellow  in  O.F.  It  gives  the  Cl  test  with  a  NaPO3 
bead  saturated  with  CuO. 


•m 


318        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Soluble  in  HN03. 

Uses.  Vanadinite  is  one  source  of  the  vanadium  used  as  alloy 
with  steel  and  of  various  compounds  used  in  dyeing  and  the 
manufacture  of  ink. 

Distinguishing  Features.  Vanadinite  is  distinguished  from 
most  minerals  by  the  adamantine  luster  and  hexagonal  crystals. 
It  is  distinguished  from  pyromorphite  by  its  color. 

Occurrence.  1.  A  secondary  mineral  formed  from  galena 
and  often  associated  with  wulfenite.  Yuma  county,  Arizona,  is  a 
prominent  locality. 

Vivianite,  Fe3(PO4)2.8H2O 

Form.  Vivianite  occurs  in  small  crystals,  in  nodules,  and  in 
earthy  masses.  Crystals  are  monoclinic  with  varying  habit. 

Cleavage,  perfect  in  one  direction  (010). 

H.  =  2.  Sp.gr.  2.6  ±. 

Color,  deep  blue  or  bluish-green,  but  colorless  if  unaltered. 

Optical  Properties.  n7(1.62)  -  w«(1.57)  =  0.05.  Fragments 
are  prismatic  with  either  parallel  or  oblique  (28J^°)  extinction. 
Pleochroic  from  blue  to  colorless  or  blue  to  green. 

Chemical  Composition.  Hydrous  ferrous  phosphate,  Fe3- 
(PO4)2-8H2O;  (H2O  =  28.7  per  cent.).  It  usually  contains  some 
ferric  iron  as  the  result  of  oxidation. 

Blowpipe  Tests.  Easily  fusible  (2)  to  a  black  magnetic  glob- 
ule. In  the  closed  tube  gives  water  and  whitens.  It  gives  the 
borax  bead  test  for  ferrous  iron.  (A  bead  made  blue  with  CuO 
becomes  opaque  red  in  a  neutral  flame.) 

Soluble  in  HNO3  or  HCL     (NH4)2MoO4  gives  a  yellow  ppt. 

Distinguishing  Features,  The  dark  blue  color  and  inferior 
hardness  are  distinctive. 

Occurrence.  1.  As  a  secondary  mineral  in  veins  associated 
with  pyrite  and  pyrrhotite.  Ibex  mine,  Leadville,  Colorado. 

2.  In  clay  and  marl  beds  sometimes  replacing  fossils  and  in 
soils  often  around  the  roots  of  trees.  At  Mullica  Hill,  New 
Jersey,  it  replaces  fossil  belemnites. 


PHOSPHATES,  NITRATES,  BORATES,  ETC. 


319 


COLLOPHANE,3Ca3(PO4)2.nCa(C03,F2)(H2O)x 

Form.  Amorphous.  Usually  massive,  often  concretionary  or 
oolitic  and  sometimes  banded.  It  is  often  a  replacement  of 
bone.  Colloform  crusts  may  be  present  in  open  spaces. 

H.  =  3  to  5.  Sp.  gr.  2.6 — 2.9  (variations  are  dependent  upon 
purity  and  porosity). 

Color,  white,  yellow,  brown,  gray,  or  black  (coloration  is  due 
to  an  organic  pigment). 

Optical  Properties,  n  =  1.58-1.62.  Fragments  are  irregular, 
colorless  to  brown,  translucent,  with  high  relief  in  clove  oil  and 
low  relief  in  cinnamon  oil.  Between  crossed  nicols  the  mineral  is 
either  dark  or  has  low  first-order  interference  colors  (the  double 
refraction  is  due  to  strain). 

Chemical  Composition.  Hydrous  calcium  carbonophosphate 
with  n  in  the  above  formula  varying  from  1  to  2.  Some  speci- 
mens approach  the  formula  of  dahllite,  3Ca3(PO4)2-CaCO3. 
Organic  matter,  aluminum,  and  iron  are  usually  present,  and 
sometimes  the  sulfate  radical.  Calcite  is  the  principal  mechani- 
cal impurity. 

The  following  analyses  give  some  idea  of  the  purer  forms  of 
cellophane: 

Analyses  of  Cellophane 


Ca 

Al 

Fe 

Mg 

P04 

CO  3 

SO  4 

F 

H20 

Misc. 

Sombrero.  . 

36  2 

0  6 

52  3 

5  4 

5  0 

Pouzillac. 

35  5 

0  1 

0  3 

50  0 

5  1 

0  9 

7  1 

03 

Nauru 

34  7 

0  5 

0  3 

1  2 

48  8 

2  5 

7  o 

01 

Alberta  (Fossil  Bone). 

34.7 

0.4 

1.4 

tr. 

50.5 

6.4 

0.1 

0.9 

3.5 

0.2 

Blowpipe  Tests.  Fuses  (at  5  to  6)  with  difficulty  on  the  edges, 
glows,  and  turns  white.  In  the  closed  tube  it  turns  dark  and 
gives  water  (1  to  8  per  cent.). 

It  is  soluble  in  cold  dilute  nitric  acid  with  slight  to  moderate 
effervescence;  when  the  acid  is  heated  there  is  vigorous  effer- 
vescence. A  hot  nitric  solution  gives  a  yellow  precipitate  with 


320        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

an  excess  of  ammonium  molybdate  solution.  The  solution 
gives  the  calcium  test  with  dilute  H2SO4  and  50  per  cent,  alcohol 
(see  p.  39  for  separation  of  calcium,  etc.). 

Distinguishing  Features.  Cellophane  is  a  difficult  mineral 
to  recognize  without  chemical  and  optical  tests  on  account  of  its 
variable  appearance.  A  mineral  showing  bone  structure  is 
almost  certain  to  be  cellophane.  It  is  distinguished  from  opal 
and  chalcedony  by  inferior  hardness,  from  calcite  by  its  greater 
hardness  and  optical  tests,  from  apatite  by  its  amorphous  nature, 
decided  effervescence  in  acids,  and  optical  tests.  It  is  apt  to  be 
overlooked  because  of  its  indefinite  character. 

Uses.  Phosphorite  or  so-called  phosphate  rock,  the  chief 
constituent  of  which  is  cellophane,  is  used  in  the  manufacture  of 
superphosphate,  a  valuable  fertilizer  made  by  treating  the  crude 
material  with  sulfuric  acid.  The  United  States  and  Tunis  are 
the  largest  producers.  The  states  in  order  of  production  are 
Florida,  Tennessee,  and  South  Carolina.  Large  deposits  occur 
in  southeastern  Idaho  and  adjoining  portions  of  Utah  and 
Wyoming  and  extend  into  Montana. 

Occurrence.  1.  In  bedded  deposits,  often  formed  by  the  re- 
placement of  limestones.  Marion  county,  Florida. 

2.  In  recent  deposits.     Nauru  Island,  Pacific  Ocean. 

3.  As  a  replacement  of  fossil  bones.     The  bone  structure  is 
retained  and  cavities  are  often  filled  with  calcite  or  chalcedony. 
The  organic  matter  has  largely  disappeared,  but  some  of  it  re- 
mains as  a  pigment. 

Turquois,  H5[Al(OH)2]6Cu(OH)(P04)4 

Form.  Turquois  occurs  in  seams  and  incrustations,  and  is 
apparently  amorphous,  but  the  polarizing  microscope  proves  it  to 
.be  crystalline  and  at  one  locality  it  has  been  found  in  minute 
triclinic  crystals. 

H.  =  6.  Sp.  gr.  2.7±. 

Color,  usually  bluish-green   but  varies  from  blue  to  green. 

Optical  Properties.     n7(1.65)  -  wa(1.61)  =  .04.     Fragments 


PHOSPHATES,  NITRATES,  BORATES,  ETC.  321 

are  irregular,  bluish  or  greenish  with  aggregate  polarization  in 
low  first-order  interference  colors. 

Chemical  Composition.  Acid  and  basic  aluminum  copper 
phosphate,  probably  H5[Al(OH2)]6-Cu(OH)(P04)4,  (H20  =  19.5 
per  cent). 

Blowpipe  Tests.  Infusible,'  but  turns  dark  when  heated  and 
gives  a  green  flame  which  is  made  blue  by  HC1.  In  the  closed 
tube  at  a  high  temperature  it  gives  water  and  turns  dark. 

Soluble  in  HC1. 

Distinguishing  Features.  The  only  common  mineral  resem- 
bling turquois  is  chrysocolla  from  which  it  is  distinguished  by 
superior  hardness.  The  association  of  chrysocolla  with  other 
copper  minerals  is  also  an  aid  in  its  identification. 

Uses.  Turquois  is  used  extensively  as  a  gem.  Blue  stones 
are  more  valuable  than  green  ones. 

Occurrence.  1.  In  seams  of  volcanic  rocks  such  as  trachytes 
and  rhyolites.  Los  Cerrillos  Mts.,  New  Mexico  (in  andesite). 

Carnotite,  K(UO2)2(VO4)2  8H2O 

Form.  An  apparently  amorphous  mineral  occurring  in  earthy 
masses,  impregnations,  or  as  incrustations.  Carnotite,  however, 
is  crystalline  and  minute  tabular  orthorhombic  crystals  have  been 
described. 

H.  =  rather  soft.  Sp.  gr.  =  4.1  ±. 

Color,  canary  yellow. 

Optical  Properties.  n7(1.95)  -  wa(1.75)  =  0.20.  Fragments 
are  yellow  and  doubly-refracting. 

Chemical  Composition.  Hydrous  potassium  uranyl  vanadate, 
K2(UO2)2(VO4)2;8H2O  (H2O  =  14.5  per  cent.).  Calcium  re- 
places part  of  the  potassium.  The  corresponding  calcium 
mineral  is  called  tyuyamunite. 

Blowpipe  Tests.     Fusible  (at  2J£)  to  a  black  non-magnetic 
slag.     It  gives  a  yellow  NaPO3  bead  in  O.F.  which  becomes  a 
fine  green  in  the  R.F.     In  the  closed  tube  it  darkens  and  gives 
water. 
21 


322        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Soluble  in  cold  dilute  acids. 

Distinguishing  Features.  Carnotite  is  easily  recognized  by 
its  bright  canary  yellow  color. 

Uses.  Carnotite  is  used  for  the  production  of  vanadium, 
uranium,  and  radium  salts. 

Occurrence.  1.  As  an  incrustation  or  impregnation  in  sand- 
stones, sometimes  associated  with  fossil  wood.  It  is  evidently 
a  secondary  mineral  derived  from  some  pre-existing  minerals. 
Western  Colorado  and  eastern  Utah. 

Nitratine,  NaNO8 

Form.  Nitratine  or  "soda  niter"  is  found  in  crystalline  and 
granular  masses.  It  crystallizes  in  rhombohedrons  with  almost 
the  same  angles  as  the  unit  or  cleavage  rhombohedron  of  calcite 
(lOTl  :0lll  =  73°  30'). 

Cleavage,  rhombohedral  like  caleite. 
H.  =  l^  to  2.  Sp.  gr.  2.3±. 

Color,  white  or  colorless.  Very 
deliquescent. 

Optical  Properties.  w7(i.587)  - 
na(1.336)  =  0.251.  It  recrystallizes  from 
water  solution  in  rhombic  shaped  crystals 
(Fig.  486)  with  symmetrical  extinction  and 

FIG.  486. — Nitratine  re-  i_-    i_        j        •    j.     £  i  i~-u 

crystallized.  very  high-order  interference  colors  which 

greatly  resemble  calcite  cleavage  fragments. 

The  indices  of  refraction,  however,  are  much  lower  than  those 
of  calcite. 

Chemical  Composition.  Sodium  nitrate,  NaN03.  lodin  may 
be  present  in  the  form  of  Ca(IO3)2,  which  imparts  a  yellow  tint. 

Blowpipe  Tests.  Easily  fusible  (at  1),  giving  an  intense  yellow 
flame.  With  KHS04  in  the  closed  tube  it  gives  red-brown  fumes 
of  NO2. 

Soluble  in  water. 

Distinguishing  Features.  Nitratine  is  distinguished  by  its 
cooling,  saline  taste,  and  by  the  optical  tests  which  are  much  like 
those  of  calcite. 


PHOSPHATES,  NITRATES,  BOKATES,  ETC.  323 

Uses.  Nitratine  is  used  as  a  fertilizer  and  also  in  the  manu- 
facture of  potassium  nitrate.  Chile  furnishes  the  world's  supply. 

Occurrence.  1.  In  superficial  beds  ("caliche")  in  arid  regions. 
Occurs  in  northern  Chile  and  to  a  slight  extent  in  California  and 
Nevada. 

2.  In  caves.     Holmdale,  Idaho. 

Colemanite,  Ca2B6Oir5H2O 

Form.  Colemanite  occurs  in  crystals,  which  often  line  geodes, 
and  in  crystalline  and  compact  masses.  The  crystals  are  mono- 
clinic  and  are  often  highly  modified. 

Cleavage,  perfect  in  one  direction  parallel  to  (010). 

H.  =  4M-  Sp.  gr.  2.4  ±. 

Color,  colorless  or  white. 

Optical  Properties.  n7(1.61)  -  na(1.58) 
=  0.03.  Fragments  are  irregular  plates 
with  bright  interference  colors.  Pseudo- 
hexagonal  crystals  of  boric  acid  separate 
from  the  hydrochloric  acid  solution  (Fig. 
487). 

Chemical    Composition.     Hydrous    cal- 
cium hexa-borate,    Ca2B6On-5H2O;    (B2O3      FIG.  487.— Boric  acid. 
=  50.9  per  cent.,  H2O  =  21.9  per  cent.). 

Blowpipe  Tests.  Fuses  easily  (at  1^£)  with  exfoliation,  coloring 
the  flame  green.  In  the  closed  tube  it  gives  water. 

Soluble  in  hot  HC1.     Boric  acid  separates  out  on  cooling. 

Distinguishing  Features.  Colemanite  is  distinguished  from 
most  minerals  by  its  perfect  cleavage  in  one  direction.  From 
gypsum  it  is  distinguished  by  its  greater  hardness. 

Uses.  Colemanite  is  the  principal  source  of  borax  and 
boric  acid.  It  is  obtained  in  San  Bernardino,  Inyo,  and  Los 
Angeles  counties,  California. 

O  ccurrence.  1 .  In  shales  and  probably  formed  by  the  replace- 
ment of  ulexite.  Calico,  San  Bernardino  county,  California. 


324        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Ulexite,  NaCaB5O9  8H2O 

Form.  Ulexite  is  found  in  rounded  fibrous  masses,  locally 
called  " cotton-balls,"  and  also  in  compact  translucent  masses. 

H.  =  1  -  3.  Sp.gr.  1.6  ±. 

Color,  white.    Luster,  silky  to  vitreous. 

Optical  Properties.  n7(1.520)  -  -  rc«(1.500)  =  .020.  Frag- 
ments are  acicular  with  parallel  extinction,  negative  elongation, 
and  low  first-order  interference  colors. 

Chemical  Composition.  Hydrous  sodium  calcium  penta- 
borate,  NaCaB5O9-8H2O;  (H2O  =  35.5  per  cent.). 

Blowpipe  Tests.  Easily  fusible  (1)  with  intumescence,  color- 
ing the  flame  an  intense  yellow.  In  closed  tube  gives  water. 

Soluble  in  hot  HC1;  H3BO3  separates  out  on  cooling. 

Uses.     It  is  a  source  of  borax  in  northern  Chile. 

Distinguishing  Features.  The  soft  rounded  masses  with 
fibrous  structure  and  silky  luster  are  characteristic. 

Occurrence.  1 .  In  playas  or  the  dried-up  lakes  of  arid  regions 
associated  with  borax,  gypsum,  and  halite.  Esmeralda  County, 
Nevada. 

Pitchblende,  (UO2)2UO4(H2O), 

Form.  Pitchblende  is  usually  massive  and  sometimes  has  a 
colloform  surface.  It  is  an  amorphous  mineral.  Uraninite  is 
the  crystalline  equivalent. 

H.  =  5K-  Sp.  gr.  6.5  to  8.0. 

Color,  dark  brown  to  black.  Luster,  submetallic  or  pitch- 
like.  Streak  usually  olive-green. 

Optical  Properties.  n>  1.74.  Fragments  are  irregular,  trans- 
lucent brown,  and  isotropic. 

Chemical  Composition.  Probably  uranyl  uranate  (UO2)sr 
U04(H2O);E.  Lead  and  radium  are  present,  the  latter  in  very 
small  quantities. 

Blowpipe  Tests.  Infusible.  The  NaPO3  bead  is  yellowish- 
green  in  O.F.  and  green  in  R.F.  It  gives  water  in  the  closed  tube. 


PHOSPHATES,  NITRATES,  BORATES,  ETC.  325 

Soluble  in  HN03.  With  NH4OH  the  solution  gives  a  yellow 
precipitate. 

Distinguishing  Features.  This  mineral  has  no  very  distinctive 
characters  outside  of  its  high  specific  gravity  and  pitchy  luster. 

Uses.  Uraninite  is  the  source  of  uranium  compounds  and  also 
of  radium  compounds. 

Occurrence.  1.  In  veins  with  metallic  sulfids.  Joachimsthal, 
Bohemia. 


10.  SULFATES 


A.  Normal  Anhydrous  Sulfates 

[BARITE,         Baso4 

Barite  Group  |  Celestite,  SrSO4 

[  Anglesite,  PbSO4 

ANHYDRITE,    CaSO4 

B.  Basic  and  Hydrous  Sulfates 


Alunite  Group 


Kainite, 

Brochantite, 

GYPSUM, 

Chalcanthite, 

Alunite, 

Jarosite, 


MgS04KC13H20 
Cu4(OH)6SO4 
CaSO42H2O 
CuSO4  6H2O 

KA13(OH)6(S04)2 
KFe3(OH)6(S04)2 


A  large  number  of  sulfate  minerals,  most  of  them  basic  and 
hydrous  salts,  are  known,  but  comparatively  few  are  of  much 
importance.  They  are  all  salts  of  H2S04.  No  sulfites,  pyrosul- 
fates,  thiosulfates,  or  persulfates  are  known  among  minerals. 

BARITE  GROUP— ORTHORHOMBIC 

In  crystal  habit,  angles,  and  cleavage  barite,  celestite,  and 
anglesite  are  similar,  and  thus  constitute  an  isomorphous  group. 
One  would  expect  to  find  anhydrite  in  this  group,  but  it  differs  in 
angles  and  especially  in  cleavage.  There  are  isomorphous  mix- 
tures of  BaSO4  and  SrSO4,  also  BaS04  and  PbS04.  The  following 
are  analyses  of  minerals  of  this  group. 

Analyses  of  Minerals  of  the  Barite  Group 


BaO 

SrO 

CaO 

PbO 

SO, 

Misc. 

Strontium-bearing  barite  
Lead-bearing  barite  (Hokutolite)  
Celestite 

43.8 
48.9 

13.9 
54  7 

0.1 

1   4 

17.8 

36.9 
32.2 
43  8 

3.7 
Ign.  -  0.6 

Anglesite  ... 

74  0 

25  7 

HjO  -  0.3 

326 


SVLFATES 


BARITE  BaSO4 


327 


Form.  Barite  occurs  in  crystals,  in  crested  groups,  in  lamellar, 
nodular,  fibrous,  and  granular  masses. 

Rhombic  bipyramidal  class:  d:  5:  6  =  0.815:1:1.313.  Usual 
forms:  c{001),  m(HQ},  o{011},  w{101j,  d{102],  1(104} t  z(lll}t 
y{  122).  Interfacial  angles:  mm  (110:110)  =  78°  22';  co  (001:011) 
=  52°  43';  cu  (001:101)  =  58°  10>^';  cd  (001:102)  =  38°  51  J£'; 
cl  (001:104)  =  21°  56^';  cz  (001:111)  =  64°  19';  oy  (011:122) 
=  26°  1'.  The  habit  is  usually  tabular  parallel  to  {001} ,  as  rep- 
resented in  Figs.  488  to  491,  but  prismatic  crystals  are  also 
common.  (See  Fig.  203,  p.  115). 


FIG.  488.  FIG.  489.  FIG.  490. 

FIGS.  488-491.— Barite. 


FIG.  491. 


Cleavage,  parallel  to  c{001}  and  to  mfllO).  The  cleavage 
form  is  like  Fig.  488,  with  two  right  angles  and  one  oblique  angle 
(78°  22r) . 

H.  =  3.  Sp.  gr.4.5±. 

Color,  colorless,  white,  gray  and  tints  of  brown,  .blue,  green,  etc. 

Optical  Properties,  wr(1.647)  -  rza(1.636)  =  0.011.  Frag- 
ments are  rhombic  with  symmetrical  extinction  or  rectangular 
with  parallel  extinction.  The  interference  colors  are  bright. 

Chemical  Composition.  Barium  sulfate,  BaS04;  (BaO  =  65.7 
per  cent.)  Strontium  and  lead  often  replace  part  of  the  barium. 

Blowpipe  Tests.  Fusible  (at  4),  coloring  the  flame  yellowish- 
green.  Unaltered  in  the  closed  tube,  but  usually  decrepitates. 
The  water  solution  of  the  sodium  carbonate  fusion  gives  a  white 
precipitate  with  BaCl2,  which  is  insoluble  in  HC1.  The  acetic 
acid  solution  of  the  residue  gives  a  yellow  precipitate  with 
K2CrO4. 

Insoluble  in  acids. 


328        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Distinguishing  Features.  Barite  can  usually  be  recognized 
by  its  lamellar  structure  and  high  specific  gravity. 

Uses.  Barite  is  used  in  the  manufacture  of  paint  as  a  substi- 
tute for  white  lead  and  also  in  the  manufacture  of  barium  salts. 

Occurrence.  1.  As  a  gangue  mineral  in  veins,  associated  with 
galena,  sphalerite,  dolomite,  calcite,  etc. 

2.  As  lenticular  masses  in  clays  overlying  limestones.     These 
are  residual  deposits  formed  by  the  weathering  of  the  limestone. 

3.  As  a  metasomatic  replacement  of  limestone  and  in  cavities 
in  limestones. 

Celestite,  SrSO4 

Form.  Celestite  occurs  in  crystals,  in  cleavable  masses,  and  in 
fibrous  seams.  The  crystals  are  like  barite  in  habit,  forms,  and 
angles.  Crystals  one  and  a  half  feet  in  length  have  been  found 
on  the  Island  of  Put-In-Bay,  Lake  Erie. 

Cleavage,  perfect  parallel  to  {001},  and  imperfect  parallel  to 
{110}. 

H.  =  3.  Sp.gr.3.9±. 

Color,  colorless,  white,  pale  blue;  sometimes  red. 

Optical  Properties.  ^(1.631)  =  n«(1.622)  =  0.009.  Frag- 
ments are  like  those  of  barite. 

Chemical  Composition.  Strontium  sulfate,  SrSCU;  (SrO  = 
56.4  per  cent.).  Calcium  and  barium  sometimes  are  present. 

Blowpipe  Tests.  Fusible  (at  4)  giving  a  crimson  red  flame 
with  HC1.  The  water  solution  of  the  sodium  carbonate  fusion 
gives  a  white  ppt.  with  BaCl2,  which  is  insoluble  in  HC1.  The 
acetic  acid  solution  of  the  residue  fails  to  give  a  precipitate  with 
K2CrO4,  but  the  addition  of  NH4OH  and  alcohol  causes  a  yellow 
ppt.  to  form. 

Insoluble  in  acids. 

Distinguishing  Features.  Celestite  greatly  resembles  barite 
but  can  often  be  distinguished  by  its  imperfect  prismatic  cleavage. 
It  is  not  as  heavy  as  barite. 

Uses.  Celestite  is  used  to  some  extent  in  the  manufacture  of 
fire-works. 


SULFATES 


329 


Occurrence.  1.  In  cavities  in  limestone.  Near  Austin, 
Texas. 

2.  In  marl  with  sulfur  and  gypsum.     Girgenti,  Sicily. 

Anglesite,   PbSO4 

Form.  There  are  two  characteristic  occurrences  of  anglesite : 
in  crystals  in  cavities,  and  in  masses  with  a  banded  structure. 
The  crystals  are  orthorhombic  and  of  varied  habit.  See  Figs. 
492-494,  in  which  ra{110(,  a{100),  c{001},  d{102|,  ojOll}, 
zjlll),  y{!22}.  The  angles  are  almost  the  same  as  for  barite. 
Unlike  cerussite  it  never  occurs  in  twin-crystals. 


m 


FIG.  492. 


FIG.  493. 
FIGS.  492-494. — Anglesite. 


FIG.  494. 


Cleavage,  imperfect  and  not  important. 

H.  =  3.  Sp.    gr.    6.3  ±. 

Color,  colorless,  white,  or  gray.     Luster,  adamantine  to  dull. 

Optical  Properties.  r?7(1.893)  -  na(1.877)  =  0.016.  Frag- 
ments are  irregular  with  bright  interference  colors  (cerussite 
has  very  high-order  colors). 

Chemical  Composition.  Lead  sulfate,  PbSO4;  (PbO  =  73.6 
per  cent.,  Pb  =  68.3  per  cent.). 

Blowpipe  Tests.  Easily  fusible  (at  lj£)  on  charcoal  to  a  white 
globule.  In  R.F.  on  charcoal  gives  a  metallic  button. 

Soluble  in  HNO3  with  difficulty.  Soluble  in  NH^CaHsOa) 
(made  by  neutralizing  acetic  acid  with  ammonium  hydroxid). 


330        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Distinguishing  Features.  In  its  general  appearance  and 
associates,  anglesite  resembles  cerussite  from  which  it  is  usually 
distinguished  by  the  similarity  of  its  crystals  to  those  of  barite 
and  by  the  absence  of  twinned  crystals.  It  also  has  weaker 
double  refraction  than  cerussite. 

Uses.     Anglesite  is  one  of  the  minor  ores  of  lead. 

Occurrence.  1.  As  a  secondary  mineral  in  the  oxidized 
zone  of  lead  mines.  It  often  accompanies  cerussite  and  is  some- 
times pseudomorphous  after  galena.  It  frequently  occurs  as 
band  surrounding  galena  and  between  it  and  cerussite.  Phoenix- 
ville,  Pennsylvania. 

ANHYDRITE,  CaSO4 

Form.  Anhydrite  occurs  in  cleavable  and  granular  masses 
and  but  rarely  in  orthorhombic  crystals. 

Cleavage,  in  three  directions  at  right  angles  (pseudo-cubic). 

H.  =  3  to  3^.  Sp.  gr.  2.9  ±. 

Color,  colorless,  white,  gray,  bluish,  or  reddish.  Luster, 
pearly  on  cleavage  faces. 

Optical  Properties.  n7(1.61)  -  n«(1.57)  =  0,04.  Fragments 
are  square  and  rectangular  with  parallel  extinction  and  bright 
interference  colors.  There  are  often  twinning  striations  parallel 
to  the  diagonals  of  the  squares.  With  dilute  HC1,  microchemical 
gypsum  is  formed  (Fig.  4,  p.  43). 

Chemical  Composition.  Anhydrous  calcium  sulfate,  CaS04; 
(CaO  =  41.2  per  cent.). 

Blowpipe  Tests.  Fuses  (at  3)  and  colors  the  flame  yellowish- 
red.  In  the  closed  tube  it  may  yield  a  little  water  due  to  partial 
hydration  to  gypsum. 

Soluble  with  difficulty  in  HC1. 

Distinguishing  Features.  Anhydrite  is  recognized  by  its 
pseudo-cubic  cleavage  and  by  its  moderate  specific  gravity 
(heavier  than  calcite  and  lighter  than  barite).  It  is  heavier 
and  harder  than  gypsum  and  only  soluble  with  difficulty  in  HC1. 

Occurrence.     1.  In  bedded  deposits  due  to  the  direct  deposi- 


SULFATES 


331 


tion  of  sea-water  and  often  associated  with  halite.     Ellsworth 
county,  Kansas. 

2.  In    veins    or    vein-like    deposits.     Beaver    county,    Utah. 

3.  In  cavities  in  limestone.     Lockport,  New  York. 

Kainite,   MgSO4KC13H2O 
Form.     Kainite  usually  occurs  in  granular  masses. 


H.  =  2y2. 


Sp.  gr.  2.1  ±. 


FIG. 


Color,  white,  colorless,  gray,  or  reddish. 

Optical  Properties.  nT(1.52)  -  n«(1.49)  =  0.03.  Recrystal- 
lizes  from  water  solution  in  the  following  order:  (1)  K2Mg (804)2' 
6H2O,  prismatic  crystals  with  oblique  ex- 
tinction. (2)  KC1,  isotropic  squares.  (3) 
MgSO4-7H20  and  MgCl2-6H20,  confused 
streaky  aggregates.  Figure  495  represents 
the  three  stages  of  crystallization.  The 
equation  is  3(M°SO4-KC1  3H2O)  +  10H2O  = 
K2Mg(SO4)2  6H20  H-  KC1  +  MgS04  7H2O 
+  MgCl2  6H2O. 

Chemical  Composition.  Hydrous  mag- 
nesium sulfate  and  potassium  chlorid. 
MgSO4-KCl-3H2O;  (H20  =  21.7  per  cent.). 

Blowpipe   Tests.     Easily  fusible    (at   2), 
violet.     In  the  closed  tube  gives  water. 

Soluble  in  water.  The  solution  gives  wet  tests  for  Mg,  SO4, 
and  Cl. 

Distinguishing  Features.  Kainite  is  distinguished  from  halite 
and  nitratine  by  the  absence  of  cleavage  and  by  the  bitter  taste. 

Uses.  Kainite  is  extensively  used  as  a  fertilizer  and  as  a 
source  of  potassium  salts.  Stassfurt,  Prussia,  is  practically  the 
only  producer. 

Occurrence.  1.  A  secondary  mineral  of  the  Stassfurt  salt 
deposits  resulting  from  the  action  of  magnesium  sulfate  on 
carnallite,  (KMgC]8-6H2O.). 


495. — Kainite 
crystallized. 


coloring  the  flame 


332         INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Brochantite,  Cu4(OH)6SO4 

Form.     Brochantite  is  found  in  small  prismatic   crystals,  in 
drusy  crusts,  and  in  fibrous  masses. 


H.  =  3K.  Sp.  gr.  3.9  ±. 

Color,  emerald  green. 

Optical  Properties.  w7(1.803)  -  -  wa(1.730)  =  0.073.  Frag- 
ments are  prismatic  with  parallel  extinction. 

Chemical  Composition.  Basic  copper  sulfate,  Cu4(OH)6SO4 
orCuSO4-3Cu(OH)2;  (Cu  =  56.2  per  cent.;  H2O  =  12  per  cent.). 

Blowpipe  Test.  Fusible  at  3^.  In  the  closed  tube  it  turns 
black  and  gives  off  water. 

Uses.    At  Chuquicamata,  Chile,  it  is  the  principal  ore  of  copper. 

Insoluble  in  water.     Soluble  in  HNOs  without  effervescence. 

Distinguishing  Features.  Brochantite  greatly  resembles 
malachite.  It  is,  however,  soluble  in  acids  without  effervescence. 
From  chalcanthite  it  is  distinguished  by  the  fact  that  it  is  not 
soluble  in  water. 

Occurrence.  1.  A  secondary  mineral  associated  with  other 
copper  minerals  in  the  oxidized  zone.  Chuquicamata,  Chile. 

GYPSUM,  CaSO42H2O 

Form.  In  form  gypsum  is  variable.  It  occurs  in  embedded 
and  attached  crystals,  in  cleavage  and  crystalline  masses,  and  in 
fibrous  and  granular  masses. 

Monoclinic  system.  Prismatic  class:  d:b:6  =  0.689:1:0.412; 
ft  =  80°  42'.  Usual  forms:  rajllO},  1(111}  ,  6{010),  e{103[. 
Interfacial  angles:  mm(HQ:  110)  =  J>8°_30';j/(lll:  111)  =  36°12'; 
mZ(110  :  111)  =  49°  9';  ae(edge  110-ITO:I03)  =  87°  49'.  The 
habit  is  usually  tabular  parallel  to  the  side  pinacoid  {  010)  .  Figs. 
496  to  498  represent  typical  crystals.  Twins  with  {  100}  as  twin- 
plane  are  common  (Fig.  498). 

Cleavage,  perfect  in  one  direction  parallel  to  {010},  also 
imperfect  conchoidal  parallel  to  (100),  and  fibrous  parallel  to 


SULFATES 


333 


{111}.  A  cleavage  fragment  is  oriented  with  respect  to  the 
crystal  outline  as  shown  in  Fig.  499. 

H.  =  2to2>^.  Sp.gr.  2.3+   . 

Color,  colorless,  white,  amber,  gray,  pink,  etc.  Luster, 
vitreous,  silky,  or  pearly. 

Optical  Properties.  n7(1.529)  -  ntt(  1.520)  =  x0.009.  Frag- 
ments are  prismatic,  acicular,  or  platy  with  bright  interference 
colors  and  extinction  angles  of  0°,  13^°,  or  37^°.  Recrystallizes 
from  dilute  HC1  solution  as  microchemical  gypsum  (Fig.  4, 
p.  43). 


m 


I 


m 


FIG.  496. 


FIG.  497.  FIG.  498. 

FIGS.  496-499. — Gypsum. 


FIG.  499. 


Chemical  Composition.  Hydrous  calcium  sulf  ate,  CaSO4'  2H2O ; 
(H2O  =  20.9  per  cent.).  Massive  gypsum  may  contain  calcite, 
anhydrite,  clay,  sand,,  or  organic  matter. 

Blowpipe  Tests.  Easily  fusible  (at  3)  to  a  white  enamel,  giving 
a  yellowish-red  flame.  In  tne  closed  tube  it  becomes  opaque  and 
gives  off  water  at  a  low  temperature. 

Easily  soluble  in  dilute  HC1  (distinction  from  anhydrite)  and 
slightly  soluble  in  water. 

Distinguishing  Features.  Gypsum  is  distinguished  from  most 
minerals  of  similar  appearance  by  its  inferior  hardness  and  low 
specific  gravity.  In  addition,  it  is  distinguished  from  anhydrite 
by  its  easy  solubility,  high  water  content,  and  by  optical  tests. 


334         INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

The  optical  tests  are  valuable,  for  gypsum  and  anhydrite  can 
be  recognized  in  the  presence  of  each  other. 

Uses.  Gypsum  is  extensively  used  as  plaster  and  as  a  fertilizer 
to  neutralize  black  alkali  (sodium  carbonate)  in  arid  regions. 
France  and  the  United  States  are  the  principal  sources  of  gypsum. 
In  this  country,  New  York,  Iowa,  Michigan,  and  Ohio  lead  in 
the  production. 

Occurrence.  1.  As  bedded  deposits  associated  with  salt  and 
limestone  and  formed  directly  by  the  evaporation  of  inland  seas. 

2.  As  a  secondary  mineral  in  various  rocks  formed  principally 
by  the  action  of  sulfuric  acid  or  ferrous  sulfate  (produced  by  the 
oxidation  of  pyrite)  on  calcium  carbonate. 

3.  As  a  hydration  product  of  anhydrite.     At  the  Ludwig  mine 
in  Lyon  county,  Nevada,  the  hydration  has  reached  a  depth  of 
400  feet. 

4.  As  gypsum  earth  (or  gypsite)  deposits  formed  by  solution  in 
fresh  water  and  reprecipitation. 

Chalcanthite,  CuSO4-5H2O 

Form.  Chalcanthite  occurs  as  an  incrustation  or  as  fibrous 
seams.  Artificial  crystals  of  this  substance  furnish  us  excellent 
examples  of  triclinic  crystals. 

H.  =  2^.  Sp.  gr.  2.2±. 

Color,  blue. 

Optical  Properties.  nY(1.54)  -  wa(1.51)  =  0.03.  Itrecrystal- 
lizes  from  water  solution  in  pale  blue  prismatic  crystals  with 
oblique  extinction  (10°  to  15°). 

Chemical  Composition.  Hydrous  copper  sulfate,  CuSO4-5H2O ; 
(Cu=  25.4  H2O  =  36.1  per  cent.).  It  often  contains  iron. 

Blowpipe  Tests.  Fusible  at  3.  In  the  closed  tube  turns  white, 
then  black,  and  yields  abundant  water. 

Soluble  in  water.  The  water  solution  placed  on  metallic  iron 
(knife-blade)  gives  a  film  of  copper. 

Uses.     In  a  few  places  it  has  been  used  in  silver  extraction- 


SULFATES  335 

Distinguishing  Features.  The  color  and  disagreeable  metallic 
taste  are  distinctive. 

Occurrence.  1.  A  secondary  mineral  often  found  in  aban- 
doned mine  drifts.  The  Bluestone  mine  in  Lyon  county, 
Nevada,  is  a  prominent  locality. 

ALUNITE  GROUP— HEXAGONAL 
The  following  minerals  form  a  well-defined  isomorphous  group : 

Alunite,  K2A16(OH)12(SO4)4 

Natroalunite,  Na2Al6(OH)12(SO4)4 

Jarosite,  K2Fe6(OH)i2(SO4)4 

Natrojarosite,  Na2Fe6(OH)i2(SO4)4 

Plumbojarosite,  PbFe6(OH)i2(SO4)4 

Carphosiderite,  H2Fe6(OH)i2(SO4)4 

These  minerals  crystallize  in  the  hexagonal  scalenohedral  class 
of  the  hexagonal  system. 

Some  rare  phosphate  and  sulfato-phosphate  minerals  are  also 
probably  isomorphous  with  the  above  listed  minerals  as  has  been 
shown  by  Schaller. 

Alunite,   KA13(OH)6(SO4)2 

Form.  Alunite  occurs  in  small  crystals  in  cavities  or  dis- 
seminated through  the  rock  mass  occasionally  in  veins  and  often 
in  fine  grained  masses.  The  crystals  belong  to  the  hexagonal 
system.  The  habit  is  usually  tabular  with  the  pinacoid  c{0001} 
and  the  rhombohedron  rjlOll)  as  represented  in  Fig.  500. 

H.  =  4.  Sp.  gr.  2.8  ±. 

Color,  colorless,  white,  gray,  or  reddish. 

Optical  Properties.  n7(1.59)  -  na(l.57)  =  0.02.  Fragments 
are  irregular  with  bright  interference  colors.  Small  crystals  are 
triangular,  dark  between  crossed  nicols  (basal  sections),  and 
give  a  positive  uniaxial  interference  figure  in  convergent  light. 

Chemical  Composition.  Basic  potassium  aluminum  sulfate, 
KA13(OH)6(S04)2  (K2O  =  11.4  per  cent,  H2O  =  13.0  per  cent.). 
Sodium  may  replace  part  of  the  potassium. 


336        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Blowpipe  Tests.  Infusible  and  turns  blue  with  cobalt  nitrate 
solution.  In  the  closed  tube  it  gives  water  which  has  an  acid 
reaction. 

Soluble  in  H2SO4  with  difficulty.  It  is  insoluble  in  water,  but 
after  roasting  it  is  converted  into  water-soluble  alum. 

Distinguishing  Features.  Alunite  is  difficult  to  recognize  at 
sight. 

Uses.     Alum  is  obtained  from  the  alunite  rock  by  roasting  and 
leaching  with  water.     It  has  recently  been  mined  near  Marys- 
vale,  Utah. 

Occurrence.  1.  A  product  of  hydro- 
thermal  alteration,  probably  brought  about 
by  ascending  acid  solutions.  Goldfield, 
Nevada. 

Jarosite,  KFe3(OH)6(SO4)2 

Form.  Jarosite  occurs  in  small  crystals 
in  cavities  or  in  massive  to  earthy  forms. 

FIG.  500. — Alunite. 

The  crystals  vary  from  hexagonal  tabular 

to  pseudo-cubic  rhombohedral  (lOTl  -IlOl)  =  90°45'.  Figure_500 
is  a  plan  of  a  typical  crystal  with  the  forms  cjOOOl  [  and  r  { lOll } . 

H  =  3.  Sp.gr.  =  3.2  + . 

Color,  yellow  to  brown. 

Optical  Properties.  w7(1.77)-w«(1.74)  =  0.03.  Fragments 
are  irregular  with  third  and  fourth  order  interference  colors,  or  are 
hexagonal  tabular  crystals  and  dark  between  crossed  nicols. 

Chemical  Composition.  Basic  potassium  ferric  sulfate  KFe3- 
(OH)6(SO4)2-(H2O  =  10.8  per  cent).  Sodium  or  hydrogen  may 
replace  part  of  the  potassium  and  thus  it  grades  into  natrojaro- 
site  and  carphosiderite. 

Blowpipe  Tests.  Fuses  with  difficulty  (at  4.5)  to  a  dark 
magnetic  mass.  In  the  closed  tube  turns  dark  and  gives  water 
which  reacts  acid  to  litmus  paper. 

Soluble  in  HC1  to  an  amber-colored  solution  which  reacts  for 
ferric  iron  and  the  sulfate  radical. 


SULFATES  337 

Distinguishing  Features.  The  blowpipe  and  wet  tests  are 
distinctive. 

Occurrence.  1.  In  the  oxidized  zone  of  ore-deposits.  Tintic 
District,  Utah. 

2.  In  sedimentary  rocks.     Near  Coalinga,  California. 


22 


11.  TUNGSTATES  AND  MOLYBDATES 


Wolframite  (Fe,Mn)WO4 

Scheelite          /  Scheelite,       CaWO4 
Group  I  Wulfenite,     PbMoO4 

WOLFRAMITE  GROUP— MONOCLINIC 


FeO 

MnO 

W03 

Ferberite  (Colorado)  

23  9 

0  7 

73  9 

Wolframite  (Burma) 

13  8 

9  4 

74  8 

Wolframite  (Zinnwald)  

9  6 

14  8 

75  6 

Htibnerite  (Colorado) 

1  6 

21  8 

76  6 

Wolframite,  (Fe,Mn)WO4 

Form.  Wolframite  occurs  in  crystals  and  in  crystalline  aggre- 
gates. The  crystals  are  monoclinic  and  are  usually  tabular 
parallel  to  {100}. 

Cleavage,  perfect  in  one  direction  parallel  to  {OlOj. 

H.  =  5to5>£.  Sp.  gr.  7.4  ±. 

Color,  black  or  dark  brown.    Luster,  sub-metallic.     Opaque. 

Chemical  Composition.  An  isomorphous  mixture  of  iron  and 
manganese  tungstates  (Fe,Mn)WO4,  varying  from  FeWO4 
(ferberite)  to  MnW04(hubnerite)  (WO3  =  76.4  per  cent.). 

Blowpipe  Tests.  Fusible  (at  3)  to  a  magnetic  globule.  The 
sodium  carbonate  fusion  is  bluish-green  (Mn). 

Soluble  in  aqua  regia  with  the  separation  of  WO3,  a  yellow  resi- 
due. 

Uses.  Wolframite  is  the  principal  source  of  tungsten.  Bur- 
ma, the  United  States,  and  Portugal  are  the  chief  producers  of 
wolframite. 

Distinguishing  Features.  Wolframite  is  distinguished  by  its 
cleavage  in  one  direction  combined  with  its  high  specific  gravity. 

338 


TUNGSTATES  AND  MOLYBDATES 


339 


Occurrence.     1.  A  vein  mineral  especially  in  tin-stone  veins 
associated  with  cassiterite,  scheelite,  etc.     Zinnwald,  Bohemia. 
2.  In  granite  pegmatites.     Black  Hills,  South  Dakota. 

SCHEELITE  GROUP— TETRAGONAL 
Besides  CaW04  (scheelite)  .and  PbMo04  (wulfenite),  there  are 
also  CaMo04  (powellite)  and  PbW04  (stolzite)  which  are  similar 
crystallographically.     Isomorphous  replacement  is  illustrated  by 
the  following  analyses: 


CaO 

PbO 

W03 

MoOs 

Misc. 

Scheelite  (Carrock  Fells)  
Scheelite  (Zinnwald)    

19.3 
20  3 

80.0 
71    1 

0.3 

8  2 

Scheelite  (Chili)        

18   1 

75  8 

CuO  =  33-  SK>2  =  07 

Wulfenite  (Eureka  Co.,  Nevada)  .  . 

1.0 

61.1 

39.3 

FezOs  =  0.4 

Scheelite,  CaWO4 

Form.  Scheelite  occurs  in  both  crystals  and  massive  form. 
Crystals  are  tetragonal  (Fig.  501)  and 
belong  to  the  tetragonal  bipyramidal 
class  (one  plane  of  symmetry  perpendic- 
ular to  an  axis  of  fourfold  symmetry). 
The  habit  is  pyramidal  with  pflll)  or 
e{  101)  as  the  dominant  form.  Angles: 
(111:111)  =  79°55H';  (101:011)  =  72° 


FIG.  501. — Scheelite. 


Cleavage,  distinct  parallel  to  {011} 
(in  four  directions). 

H.  =  4Kto5.  Sp.gr.  6.0  + . 

Color,   white,   gray,   or  pale   colors. 
Luster,  sub-adamantine. 

Optical  Properties.  r?T(1.93)  -  na(1.92)  =  0.01.  Fragments 
are  irregular  with  bright  interference  colors. 

Chemical  Composition.  Calcium  tungstate,  CaW04;  (WOa  = 
80.6  per  cent.).  The  tungsten  is  often  partially  replaced  by 
molybdenum.  The  copper  in  the  third  analysis  above  is  due  to 
partial  alteration  to  cuprotungstite  (CuWO4-2H2O). 


340        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

0 

Blowpipe  Tests.  Fusible  with  difficulty  (at  5).  The  NaPO3 
bead  is  blue  in  R.F. 

Decomposed  by  HC1  with  the  separation  of  a  yellow  residue 
(WO8)  soluble  in  NH4OH. 

Distinguishing  Features.  Scheelite  is  recognized  by  its  high 
specific  gravity  and  sub-adamantine  luster. 

Uses.  Scheelite  is  one  of  the  prominent  ores  of  tungsten 
which  is  used  to  harden  steel. 

Atolia,  San  Bernardino  county,  California,  is  the  chief  producer 
of  scheelite. 

Occurrence.  1.  In  veins,  especially  in  tin-stone  veins  with 
cassiterite,  fluorite,  topaz,  etc. 

Wulfenite,  PbMoO4 

Form.     Wulfenite  usually  occurs  in  crystals  which  are  tetra- 
gonal and  usually  tabular  in  habit.     Figure  502  is  a  plan  of  the 
common  type  of  crystal  with  cjOOl }  and  u{  102} . 
H.  =  3  Sp.  gr.  6.7+ . 

Color,    yellow,    orange,    or    red.     Luster, 
adamantine. 

Optical    Properties.     n7(2.40)  --  n«(2.30)   = 
0.10.     Fragments  are  irregular  and  yellow  with 
rather  high  interference  colors.     Thin  tabular 
F  T  Wulfenite2  ~~    crystals   give  a  negative  uniaxial    interference 

figure  in  convergent  light. 

Chemical  Composition.    Lead  molybdate,  PbMo04;  (MoO3  = 
39.3  per  cent.). 

Blowpipe  Tests.     Easily  fusible  (at  2)  on  charcoal,  giving  a 
metallic  button.     The  NaPO3  bead  is  green  in  R.F. 
Decomposed  by  HC1. 

Distinguishing   Features.     Wulfenite  is  distinguished  .by  its 
tabular  crystals,  yellow  to  red  color,  and  adamantine  luster. 
Uses.     Wulfenite  is  one  of  the  sources  of  molybdenum. 
Occurrence.     1.  In  the  oxidized  zone  of  veins  often  associated 
with  vanadinite.     Yuma  county,  Arizona. 


12.  SILICATES 


Feldspars 


Feldspathoid 
Group 


Pyroxene 
Group 


Amphibole 
Group 


GARNET 

Olivine 
Group 


PLAGIOCLASE 

/Ab  =  NaAlSi3O8\ 
\An  =  CaAl2Si2O8  / 


PYROXENE 


ORTHOCLASE,  (K,Na)AlSi3O8 

Adularia,  KAlSi3O8. 

Microcline,  KAlSi3O8 

Albite,  Abioo  to  Ab9o 
Oligoclase,  Ab90Abio  to  Ab7o  An3o 
Andesine,  Ab70  An30  to  Ab50  An50 
Labradorite,  Abso  An5o  to  Ab20  An7o 
Bytownite,  Ab30  An70  to  Abio  An90 
Anorthite,  AbioAbgo  to  Anioo 

Leucite,  KAl(SiO3)2 

Nepheline,  (Na,K)AlSiO4-(NaAlSi3O8)* 

Sodalite,  Na4AI3Cl(SiO4)3 

Lazurite,  Na5Al3S(SiO4)3 

Enstatite,  MgSiO3 

Hypersthene,  (Mg,Fe)SiO3 

Diopside,  Ca(Mg,  Fe)(SiO3)2 
Augite  m  CaMg(SiO3)2  +  n(Mg,Fe) 

(Al,Fe)2Si206 

Rhodonite,  MnSiO3 

Anthophyllite,  (Mg,Fe)SiO3 

Tremolite,  Ca(Mg,Fe)3(SiO3)4 

HORNBLENDE,  mCa(Mg,Fe)3(SiO3)4  + 

n(Al,Fe)(F,OH)SiO3 

Glaucophane,  NaAl  (SiO  3)  2 •  (Fe,  Mg)  SiO  3 

Beryl,  Be3Al2(SiO3)5 

Wollastonite,  CaSiO3   ... 

Spodumene,  LiAl(SiO3)2 

Grossularite,  Ca3Al2(SiO4)3 

Almandite,  Fe3Al2(SiO4)3 

Pyrope,  Mg3Al2(SiO4)3       ' 

Andradite,  Ca3Fe2(SiO4)3 

OLIVINE,  (Mg,Fe)2Si04 

Forsterite,  Mg2SiO4 

WiUemite,  Zn2SiO4 

CALAMINE,  Zn2(OH)2SiO3 

Scapolite,  m(3CaAl2Si2O8  CaCO3)  +  n(3NaA!Si3O8-NaCl) 
341 


342        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Epidote 
Group 


Vesuvianite,  Ca6Al3(OH,F)(SiO4)6 

Zircon,  ZrSiO4 

Topaz,  Al2(F,OH)2SiO4 

Andalusite,  Al2SiO5 

Kyanite,  Al2SiO5 

Sillimanite, 


f  EPIDOTE,  Ca2(Al,Fe'")3(OH)(SiO4)3 
\  Clinozoisite,  Ca2Al3(OH)(SiO4)3 

Prehnite,  H2Ca2Al2(SiO4)3 

Staurolite,  FeAl5(OH)(SiO6)s 

TOURMALINE,  R9Al3B2(OH)2Si4O19 

Datolite,  CaB(OH)SiO4 

Axinite,  HCa2(Fe,Mn)Al2B(SiO4)4 

MUSCOVITE,  H2KAl3(SiO4)3 

Sericite,  H2KAl3(SiO4)3 
Mica  Group     i  Lepidolite,  LiKAl2(OH,F)(SiO3)3 

BIOTITE,  (HK)2(Mg,Fe)Al2(Si04)3 
[  Phlogopite,  H2KMg3Al(SiO4)3 

CHLORITE,  H8(MgFe)6Al2(SiO6)3 

ANTIGORITE,  H4Mg3Si2O9 

Chrysotile,  H4Mg3Si2O9 

TALC,  H2Mg3(Si03)4 

Chondrodite,  Mg6(F,OH)2(SiO4)2 

Kaolinite,  H4Al2Si2O9 

Halloysite,  H4Al2Si2O9  (H2O)X 

Garnierite,  H2(Ni,Mg)SiO4  H2O 

CHRYSOCOLLA,  CuSiO32H2O? 

Glauconite,  K  Fe///(SiO3)2-(H2O)x? 

ApophyUite,  (H,K)2Ca(SiO3),  H2O 

Heulandite,  H4CaAl(SiO3)  3H2O 

Stilbite,  H4(Ca,NaKAl2(SiO3)6-4H2O 
ZEOLITES    I  Chabazite,  (Ca,Na2)Al2(SiO3)4  6H2O 
I  Analcite,  NaAl(SiO3)2  H2O 
(  Natrolite,  Na2Al2Si3Oi0-2H2O 

Titanite,  CaTiSiO5 

About  a  fourth  of  the  known  minerals  are  silicates,  though 
many  of  them  are  very  rare.  They  are  the  most  important  rock- 
forming  minerals  and  thus  make  up  the  bulk  of  the  earth's 
outer  shell.  Among  the  important  rock-making  minerals  are  the 
feldspars,  the  pyroxenes,  the  amphiboles,  and  the  micas.  These, 


SILICATES  343 

together  with  quartz,  constitute  about  87  per  cent,  of  the  earth's 
outer  shell,  according  to  F.  W.  Clarke. 

Many  of  the  silicates  are  complex  in  composition  and  the 
establishment  of  chemical  formulae  of  some  of  them  has  baffled 
the  skill  of  many  eminent  chemists.  In  a  chemical  discussion 
of  the  silicates  the  starting  point  is  H4SiO4,  which  is  called  ortho- 
silicic  acid.  The  compound  H2SiO3,  derived  thus  (H4SiO4  — 
H20  =  H2SiO3),  is  called  metasilicic  acid.  A  large  number  of 
orthosilicates  and  metasilicates  are  known  among  minerals,  but 
many  silicates  cannot  be  placed  in  either  of  these  divisions;  so 
the  assumption  has  been  made  that  other  silicic  acids  are  possible. 
Among  them  are  H6Si2O7  (2H4Si04  —  H2O),  diorthosilicic  acid; 
H2Si2O5  (H2SiO3  +  SiO2),  dimetasilicic  acid.  In  a  similar  way 
H4Si3O8,  H8Si3O]0,  H6SiO5,  and  H]0Si209  may  be  derived.  Min- 
erals corresponding  to  all  these  acids  are  known. 

Another  method  of  nomenclature  formerly  used  by  chemists 
and  still  employed  by  metallurgists  is  based  upon  the  ratio  of  the 
oxygen  of  silica  to  that  of  the  bases.  R2SiO4  may  be  written 
2ROSiO2.  Here  the  oxygen  ratio  is  1  :  1,  so  orthosilicates  are 
called  unisilicates.  Metasilicates,  RSiO3  or  ROSi02,  are  called 
bisilicates.  Polysilicates  have  the  formula  ROnSi02  and  sub- 
silicates,  nRO-SiO2,  where  n  is  greater  than  2. 

The  difficulty  of  assigning  formulae  to  many  silicate  minerals 
lies  in  the  fact  that  it  is  often  impossible  to  decide  upon  the 
valence  and  grouping  of  the  basic  elements.  Many  silicates  give 
water  when  heated  in  a  closed  tube,  but  it  is  often  difficult  and 
sometimes  impossible  to  determine  whether  hydroxyl  (OH), 
hydrion  (H),  or  so-called  water  of  crystallization  (H2O)  is  present. 
H2Zn2SiO5  is  the  empirical  formula  for  the  mineral  calamine. 
It  may  be  an  acid  oxy-orthosilicate,  H2(Zn20)SiO4,  a  basic 
metasilicate,  Zn2(OH)2Si03,  or  an  acid  salt  of  H6Si05,  one  of  the 
possible  silicic  acids. 

The  silicates  are  treated  as  far  as  possible  in  more  or  less  well- 
defined  groups.  Because  of  their  importance  the  feldspars  are 
given  first. 


344        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


FELDSPARS 

A  series  of  monoclinic  and  triclinic  silicates  of  aluminum 
with  either  potassium,  sodium,  or  calcium,  collectively  known 
as  the  feldspars,  forms  the  most  important  group  of  rock-form- 
ing minerals.  In  fact  feldspar  is  the  most  abundant  substance 
of  which  we  have  direct  knowledge,  for  it  constitutes  about  60 
per  cent,  of  the  solid  crust  of  the  earth.  The  feldspars  are 
non-metallic  minerals  with  a  hardness  of  about  6,  and  cleavage 
in  two  directions  at,  or  nearly  at,  right  angles,  and  a  specific 
gravity  of  2.6-2.7.  The  important  feldspars  are:  orthoclase, 
and  microcline,  which  are  polymorphous  forms  of 
and  plagioclase,  which  is  an  isomorpKous  mixture  of 
NaAlSi3O8  and  CaAl2Si2O8. 

The  following  are  typical  analyses  of  the  various  feldspars: 

Analyses  of  the  Feldspars 


K20 

Na20 

CaO 

A120S 

Si02 

Misc. 

Ort 
Ad 
Mi 

hoclase 

11.7 
14.0 
13.5 

4.3 
1.0 
1.6 

0.5 
1.3 

18.8 
17.9 
19.6 

64.6 
65.7 
64.8 

BaO  =  0.4;  ign.   =  0.1 
Fe2Os  =  tr. 
ign.  =  0.2 

ilaria       

jrocline  

|  Plagioclase 

Albite  

0.5 
1.3 
1.0 
tr 
0.6 

11.1 
8.5 
6.2 
4.4 
1.8 
0.2 

0.4 
4.8 
8.1 
12.0 
16.1 
19.3 

19.3 
23.8 
26.6 
29.6 
31.1 
36.8 

68.8 
61.3 
58.0 
54.2 
46.9 
44.0 

FezOs  =  0.1 
Fe2Os  =  0.4 

MgO  =  0.1;  ign.  =  0.1 
Fe203  =  1.3;  H2O  =  1.0 
MgO  =  0.2;  ign.  =  0.1 

Andesine       

Labradorite  
Bytownite  

ORTHOCLASE,  (K,Na)AlSi3O8) 

Form.  Orthoclase  occurs  in  attached  and  embedded  crystals, 
in  cleavable  masses,  and  disseminated  through  rock  masses. 
The  crystals  furnish  one  of  the  best  examples  of  the  monoclinic 
prismatic  class.  Axial  ratio:  d  :b  :6  =  0.658  :  1  :  0.555;  (3  = 
63°  57'.  Usual  forms:  cfOOl),  6J010},  zjlOlj,  ?/{201),  m{110), 
z{130},  ojTll),  rcj021).  Interfacial  angles:  bc(010  : 001)  = 
90°  0';  mm(110  :  110)  =  61°  13';  mc(110  : 001)  =  67°  47';  ex- 


SILICATES 


345 


(001  : 101)  =  50°  16M';  q/(001  :  201)  =  80°  18';  mz(110  :  130) 
=  29°  59^';  6o(010  :  111)  =  63°  8';  cn(001  :  021)  =  44°  56^'. 
The  habit  is  usually  elongate  in  the  direction  of  the  a-axis  (Figs. 
503-505),  or  elongate  in  the  direction  of  the  c-axis  and  tabular 
parallel  to  {010}  (Fig.  506). 


FIG.  503. 


FIG.  504.  FIG.  505. 

FIGS.  503-506. — Orthoclase. 


There  are  three  common  twinning  laws  for  orthoclase:  (1) 
the  Carlsbad  law  in  which  the  c-axis  is  the  twin-axis  (usually 
penetration  twins  with  b{  010}  as  the  composition  face)  (Fig.  507), 
(2)  the  Baveno  law  in  which  n{021}  is  the  twin-plane  (Fig.  508), 


m 


FIG.  507. 


FIG.  508. 
FIGS.  507-509.— Orthoclase  twins. 


FIG.  509. 


and  (3)  the  Manebach  law  in  which  cfOOlj  is  the  twin-plane 
(Fig.  509). 

Cleavage  in  two  directions  at  right  angles,  parallel  to  {  001  }  and 
{010}.  There  is  also  imperfect  cleavage  (or  parting)  parallel  to 
{110}  which  assists  in  orienting  cleavages  and  imperfect  crystals. 

H.  =  6.  Sp.  gr.  2.57  ±. 


346         INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Color,  white,  colorless,  gray,  pink,  red.     It  sometimes  shows  a 
play  of  colors. 

Optical  Properties.  rcT(1.526)  -  njl.519)  =  0.007.  Frag- 
ments are  plates  usually  with  one  set  of  parallel  straight  edges 
(Fig.  510).  Extinction  on  001  =  0°;  on  010  =  5°.  The  inter- 
ference colors  are  middle  first-order  (gray  and  straw-yellow). 
Chemical  Composition.  Potassium  aluminum  trisilicate,  (K, 
Na)AlSi308;  (K20  =  16.9,  A12O3  =  18.4,  Si02  =  64.7inKAlSi308.) 
Sodium  replaces  part  of  the  potassium. 

Blowpipe  Tests.     Fusible  with  difficulty  (5) .    When  fused  with 
powdered  gypsum  on  platinum  wire  it  gives  a  violet  flame. 
'  Insoluble  in  ordinary  acids. 
Distinguishing  Features.     Distinguished 
from  other  minerals  by  rectangular  clea- 
vage and  from  minerals  of  the  plagioclase 
group  by  the  absence  of  twinning  striations. 
Its  distinction  from  microcline  is  difficult 
without  optical  tests. 

Occurrence.     1.  In  acid  plu tonic  igneous 
rocks,  especially  granites  and  syenites. 

2.  In  acid  volcanic  igneous  rocks,  rhyo- 
lites  and  trachytes.     This  is  usually  sani- 
dine,  a  transparent  variety  of  orthoclase. 

3.  In  certain  rare  basic  igneous  rocks.- 

4.  In  gneisses,  partly  as  a  remnant  of  igneous  rocks,  partly 
recrystallized. 

5.  In  arkoses  or  feldspathic  sandstones  (Portland,  Connecti- 
cut) and  in  some  beach  sands  (Pacific  Grove,  California). 

Adularia,  KAlSi3O8 

Form.  Adularia  occurs  in  distinct  monoclinic  crystals  with 
practically  the  same  forms  and  interfacial  angles  as  orthoclase. 
The  habit  of  the  crystals  is  usually  pseudo-orthorhombic  because 
of  the  equal  development  of  c{001[  and  zjTOl)  (see  Fig.  511) 
and  as  a  consequence  the  cross  section  is  rhombic. 


FIG.    510.— Orthoclase 
cleavage  fragments. 


SILICATES  347 

Cleavage  in  two  directions  at  right  angles,  parallel  to  { 001 }  and 
{010}. 

H.  =  6  Sp.  gr.  2.57  ±. 

Color,  colorless  or  white. 

Optical  Properties.  ^7(1.524)  -  ntt(1.518)  =  0.006.  Frag- 
ments are  plates  with  one  set  of  parallel  edges  with  extinction 
angles  of  0°  on  001,  and  5°  on  010. 

Chemical  Composition.  Potassium  aluminum  silicate  KA1- 
Si308  (K20  =  16.9  per  cent. ;  A12O3  =  18.4;  Si02  =  64.7).  Sodium 
is  practically  absent  which  fact  distinguishes  adularia  from 
orthoclase. 

Blowpipe  Tests.  Fusible  with  difficulty  (at  5) 
to  a  colorless  glass. 

Insoluble  in  ordinary  acids. 

Distinguishing  Features.  Adularia  can  usually 
be  distinguished  from  orthoclase  by  the  crystal 
habit.  The  crystals  have  a  rhombic  cross  section  Fl<?  ,. 

*  Adularia. 

as   the    6{010j    face   is   absent    or   very   narrow. 

Adularia  is  usually  colorless  and  transparent,   and  orthoclase 

translucent. 

Uses.  A  variety  of  adularia  known  as  moonstone  on  account 
of  its  beautiful  internal  reflections  is  used  as  a  gem.  It  is 
obtained  in  Ceylon. 

Occurrence.  1.  As  a  vein  mineral  formed  at  a  comparatively 
low  temperature  in  deposits  near  the  surface  which  are  gold-  or 
silver-bearing.  Guanajuato,  Mexico. 

2.  In  cavities  and  seams  of  schists  and  gneisses.  Zillerthal, 
Tyrol. 

Microcline,  KAlSi3O8 

Form.  In  crystal  form  microcline  is  almost  like  orthoclase,  but 
it  is  triclinic  with  the  angle  (001  :  010)  7=  89°  30'  instead  of  90°. 

Cleavage.  In  two  directions  at  practically  right  angles  (89° 
30'). 

H.  =  6.  Sp.  gr.  2.5  ±. 


348    INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Alblte 
Microcline 


Color,  white,  gray,  reddish,  green.  The  green  variety  is  called 
amazon-stone. 

Optical  Properties.  n7(  1.529)  -  na(1.522)  =  0.007.  Extinc- 
tion on  001  =  15°;  on  010  =  5°.  Fragments  are  plates  with 
middle  first-order  interference  colors,  and  are  usually  dis- 
tinguished from  orthoclase  by  the  "gridiron"  structure  caused 
by  polysynthetic  twinning  in  two  directions  at  right  angles. 

Chemical  Composition.  Potassium  aluminum  trisilicate  KA1- 
Si3O8,  usually  intergrown  with  albite,  NaAlSi3O8  (Fig.  512). 
This  intergrowth  which  is  known  as  perthite  or  microperthite  is 
probably  formed  by  the  unmixing  of  the  solid  solution,  (K,Na)- 

AlSi3O8,   brought    about    by    a 
lowering  of  the  temperature. 

Blowpipe  Tests.     The  same  as 
for  orthoclase. 

Uses.     The  feldspar  used  in 
the    manufacture    of    porcelain 
and  pottery  is  largely  the  micro- 
cline-albite   intergrowth   known 
FIG.   512. — Microcline-aibite  as    perthite.     Maine,    North 

intergrowth  (perthite).  Carolina,  and  Pennsylvania  are 

the  principal  producers. 

Distinguishing   Features.     Microcline    can    often   be    distin- 
guished from  orthoclase  by  the  fact  that  it  is  intergrown  with 
albite.     Otherwise   it   must   be  distinguished  by  optical  tests. 
Occurrence.     1.  In  granite  pegmatites.     Near  Florissant,  Colo- 
rado. 

2.  In  granites  (but  not  in  rhyolites). 

3.  In  gneisses. 

PLAGIOCLASE 

The  plagioclase  groups  of  feldspars  constitutes  perhaps  the  best 
defined  isomorphous  group  to  be  found  among  minerals.  There 
is  a  perfect  gradation  in  properties  from  the  albite  end  of  the 
group  with  the  formula  NaAlSi3O8  to  the  anorthite  end  of  the 


SILICATES 


349 


group  with  the  formula  CaAl2Si2Os.     Intermediate  members  of 
the  group  are  designated  by  AbTOAnn;  Ab  denotes  the  albite 


FIG.  513. — Albite  twinning. 


FIG.  514. — Pericline  twinning. 


001 


001 


molecule  and  An,  the  anorthite  molecule.  Crystals  are  triclinic, 
but  with  angles  near  those  of  orthoclase.  The  angle  (001: 
010),  for  example, .  varies  from 
86°  24'  for  albite  to  85°  50'  for 
anorthite,  while  for  orthoclase  the 
corresponding  angle  is  90°  0'.  The 
plagioclases  have  good  cleavage 
parallel  to  {001},  and  fair  cleav- 
age parallel  to  {010}.  There  is 
also  imperfect  cleavage  parallel  to 
{110}  and  fllO}. 

Twinning  is  rarely  absent  in  the 
plagioclases.  The  most  common 
twinning  is  known  as  albite  twin- 
ning, in  which  {010}  is  the  twin- 
plane.  This  is  usually  poly- 
synthetic  and  the  twin  striations, 
which  are  always  parallel  to  the 
(001:010)  edge,  (Fig.  513)  show 
best  on  the  {001}  cleavage  face.  Figure  515  shows  in  plan  and 
elevation  a  cleavage  fragment  of  plagioclase  twinned  on  the  albite 
law.  The  narrow  planes  marked  001  are  placed  at  angles  of  about 


010 


FIG.  515. — Plan  and  elevation  of 

°f 


350        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


8°  with  the  001  planes.  In  pericline  twinning,  which  is  also 
polysynthetic,  the  6-axis  is  the  twin-axis.  This  kind  of  twinning 
shows  best  on  the  {010}  cleavage  (Fig.  514),  and  the  angle  the 
striations  make  with  the  (001 :  010)  edge  differs  for  the  various 
plagioclases.  This  angle  is  given  in  the  column  of  the  following 
tabulation  labeled  " angle  of  rhombic  section."  In  optical 
properties  the  plagioclases  are  similar.  They  are  all  biaxial  with 
2V  varying  from  77°  to  90°.  The  indices  of  refraction  vary  from 
1.536  for  albite  to  1.588  for  anorthite.  The  double  refraction  is 
rather  weak  (0.007  to  0.012).  Typical  analyses  are  given  on 
page  344.  The  following  tabulation  illustrates  the  continuous 
variation  in  properties.  The  positive  angles  are  clockwise  and 
negative  angles  counter-clockwise. 


Extinc- 

Extinc- 

Angle of 

ny 

na 

tion  on 

tion  on 

rhombic 

Sp. 

001 

010 

section 

gr. 

Albite  {Ab   . 
01igoclase..{Ab9Am 
Ande8ine...{Ab7An' 
Labradorite{AblAni 
Bytownite      A°'Am 
Anorthite..    Ab'An' 

.536 
.541 
.552 
.562 
.572 
.582 
.588 

.525 
.532 
.545 
.555 
.563 
.571 
.576 

+   2H 
0 

-14 
-31 
-40 

+  23° 
+  15M 
0 
-16 
-27 
-36 
-37H 

+  22° 
+  10 
+  3 
-    1 
-   9 
-10 
-16 

2.62 
2.64 
2.66 
2.69 
2.73 
2.74 
2.76 

Albite,  Ab  (NaAlSi3O8)  to  Ab90Ani0 

Form.  Albite  occurs  in  small  crystals,  in  lamellar  masses,  and 
intergrown  with  microcline.  The  crystals  are  triclinic,  pinacoidal 
class,  and  are  usually  tabular  parallel  to  {010} .  The  usual  forms 
are  the  same  as  for  orthoclase.  Figure  513  represents  the  common 
type  of  crystal  with  cjOOl},  6{010[,  raj  110),  Af{lTO},andz|T01}. 
Albite,  pericline,  and  Carlsbad  twins  are  all  common  and  some- 
times two  or  more  of  these  are  combined  on  one  crystal. 

Cleavage,  perfect  parallel  to  (001)  and  less  perfect  parallel 
to  {010}. 

H.  =  6.  Sp.  gr.  2.62  ±. 


SILICATES  351 

Color,  white,  colorless,  or  gray. 

Optical  Properties.  n7(1.536  to  1.541)  -  na(1.525  to  1.532)  = 
0.009.  Fragments  are  plates  with  middle  first-order  interference 
colors  and  extinction  angles  of  about  3°  on  (001)  and  15°  to  23° 
on  (010).  The  index  of  refraction  is  less  than  oil  of  cloves. 

Chemical  Composition.  Abioo  to  Ab9o  Ani0;  (for  NaAlSi3O8 
Na2O  =  11.8,  A12O3  =  19.5,  SiO2  =  68.7).  A  little  calcium  is 
usually  present  as  albite  grades  into  oligoclase. 

Blowpipe  Tests.  Fusible  (at  4)  to  a  colorless  glass  and  colors 
the  flame  yellow. 

Insoluble  in  ordinary  acids. 

Distinguishing  Features.  Albite  may  resemble  barite  but  is 
distinguished  by  its  greater  hardness.  From  the  other  plagio- 
clases  it  is  only  safely  distinguished  by  optical  tests. 

Occurrence.  1.  In  granite  pegmatites  associated  with  tour- 
maline, lepidolite,  spodumene,  etc. 

2.  In  veins  and  seams  especially  in  the  hydrothermal  meta- 
morphic  rocks. 

3.  In  certain  soda-rich  igneous  rocks,  usually  intergrown  with 
microcline.     This  intergrowth  is  known  as  perthite. 

Oligoclase,  Ab90Ani0  to  Ab7oAn30 

Form.  Oligoclase  occurs  in  cleavable  masses  and  disseminated 
through  rock  masses,  but  unlike  albite  is  rarely  found  in  distinct 
crystals. 

Cleavage,  perfect  parallel  to  { 001 J ,  less  perfect  parallel  to  { 010 } . 

H.  =  6.  Sp.  gr.  2.65  ±  . 

Color,  white,  colorless,  greenish,  or  reddish. 

Optical  Properties.  wY(1.641  to  1.552)  -  na(1.532  to  1.545)  = 
0.008.  Fragments  are  plates  with  middle  first-order  interference 
colors  and  extinction  angles  of  about  3°  to  0°  (001)  and  about 
153/£  to  0°  (010).  The  fragments  usually  show  polysynthetic 
twinning. 

Chemical  Composition.  Sodium  and  calcium  aluminum 
silicate,  AbgoAnjo  to  AbyoAnso. 


352        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Blowpipe  Tests.     Fusible  at  4. 

Insoluble  in  ordinary  acids. 

Distinguishing  Features.  Distinguished  from  other  plagio- 
classes  by  optical  tests  (or  by  a  quantitative  chemical.  analysis). 

Occurrence.  1.  In  acid  and  intermediate  igneous  rocks,  such 
as  the  granite-rhyolite  series  and  the  diorite-andesite  series. 
Strange  to  say,  oligoclase  is  more  common  in  granites  than  albite. 


Andesine,  Ab70An3o  to 

Form.  Andesine  occurs  in  cleavable  masses  or  disseminated 
crystals  through  rock  masses.  Distinct  loose  crystals  are  rare. 

Cleavage,  in  two  directions  at  angles  of  86°  14'. 

H  =  6.  Sp.  gr.  2.69  ±. 

Color,  colorless,  white,  and  various  tints. 

Optical  Properties.  n7(1.552  to  1.562)  -n«(1.545  to  1.555) 
=  .007.  Fragments  are  plates  with  middle  first-order  interfer- 
ence colors  and  extinction  angles  of  0  to  4J^°  on  (001)  and  0° 
to  16°  on  (010).  They  usually  show  polysynthetic  twinning. 

Chemical  Composition.  Sodium  and  calcium  aluminum  sili- 
cate, AbroAnao  to  Ab5oAn5o. 

Blowpipe  Tests.     Fusible  (at  4).     Insoluble  in  ordinary  acids. 

Distinguishing  Features.  Distinguished  from  the  other  plagio- 
clases  by  optical  tests  (or  by  a  quantitative  chemical  analysis)  . 

Occurrence.  1.  In  intermediate  igneous  rocks  such  as  diorites 
and  andesites. 

Labradorite,  Ab6oAn5o  to  Ab30An7o 

Form.  Labradorite  occurs  in  embedded  crystals  and  in 
cleavable  masses,  but  very  rarely  in  distinct  loose  crystals.  Albite 
twinning  is  the  common  kind  of  twinning. 

Cleavage,  perfect  parallel  to  {001},  less  perfect  parallel  to 
{010}. 

H.  =  6.  Sp.  gr.  2.71  ±. 

Color,  gray  or  white,  often  showing  a  play  of  colors  which  is 
an  optical  effect  due  to  minute  inclusions. 


SILICATES  353 

Optical  Properties.  ^(1.562  to  1.572)  -  na(l.555  to  1.563)  = 
0.009.  Fragments  are  plates  with  middle  first-order  interference 
colors  and  extinction  angles  of  —  4J^0  to  —14°  (001)  and  — 16°  to 
—  27°  (010) .  The  fragments  usually  show  poly  synthetic  twinning 
and  often  minute  inclusions  arranged  in  rows. 

Chemical  Composition.  Calcium-sodium  aluminum  silicate, 
Ab5oAn5o  to  AbaoAn7o. 

Blowpipe  Tests.     Fusible  at  4. 

Soluble  with  difficulty  in  HC1. 

Distinguishing  Features.  Labradorite  is  distinguished  from 
orthoclase  by  the  twinning  striations  on  the  cleavage  surfaces. 

Uses.     Labradorite  rock  is  used  as  an  ornamental  stone. 

Occurrence.  1.  In  basic  igneous  rocks  such  as  gabbros,  dia- 
bases, and  basalts,  associated  with  olivine,  augite,  hypersthene, 
ilmenite,  and  magnetite. 

2.  As  the  principal  constituent  of  anorthosite,  a  basic  plutonic 
igneous  rock  composed  practically  of  labradorite.  Adirondack 
Mts.,  New  York. 

Bytownite,  AbsoAnyo  to  Abio  Ango 

Form.  In  cleavable  masses  and  disseminated  through  rock  like 
the  other  plagioclases. 

Cleavage,  in  two  directions  at  angles  of  about  86°. 

H.  =  6.  Sp.  gr.  2.74  ±. 

Color.     Colorless,  white,  or  gray. 

Optical  Properties.  ny  (1.572  to  1.582) -na(1.563  to  1.571) 
=  .010.  Fragments  are  plates  with  upper  first-order  inter- 
ference colors  and  extinction  angles  of  14°  to  31°  on  (001)  and 
27°  to  36°  on  (010). 

Chemical  Composition.  Calcium  and  sodium  aluminum  sili- 
cate, AbaoAnro  to  Abio  An9o. 

Blowpipe  Tests.     Fusible  at  4^. 

Slightly  soluble  in  HC1. 

Distinguishing   Features.     Bytownite    can    be    distinguished 

23 


354    INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

from  labradorite  only  by  optical  tests  Cor  by  a  quantitative  chem- 
ical analysis). 

Occurrence.     1.  In  basic  igneous  rocks  especially  gabbros. 

Duluth,  Minnesota. 

Anorthite,  AbioAn9o  to  An  (CaA^SiOs) 

Form.  Anorthite  occurs  in  cleavable  masses  and  occasionally 
in  euhedral  crystals. 

Cleavage.  Like  the  other  plagioclase  feldspars,  in  two  direc- 
tions at  angles  of  about  86°. 

H.  =  6.  Sp.  gr.  2.76 ±. 

Color,  white,  gray,  or  reddish. 

Optical  Properties.  n7(1.582  to  1.588) -n«(1.571  to  1.576)  = 
.012.  Fragments  are  plates  with  upper  first-order  interference 
colors  and  extinction  angles  of  31°  to  40°  on  (001)  and  36°  to 
37^°  on  (010). 

Chemical  Composition.  Calcium  aluminum  silicate,  usually 
with  a  little  sodium,  Abio  An9o  to  An  I0o.  (For  An,  CaO  =  20.1 ; 
A12O3  =  36.7;  SiO2  =  43.2.) 

Blowpipe  Tests.     Fusible  at  4%. 

Slowly  soluble  in  HC1  and  gives  a  jelly  of  silicic  acid  upon 
evaporation. 

Distinguishing  Features.  Optical  tests  (or  a  quantitative 
chemical  analysis)  are  necessary  to  distinguish  anorthite  from 
other  members  of  the  plagioclase  group. 

Occurrence.  1.  A  comparatively  rare  mineral  in  basic  igneous 
rocks.  Miyake,  Japan. 

FELDSPATHOID  GROUP 

Leu  cite,  nepheline,  sodalite,  and  lazurite  are  collectively  known 
as  feldspathoids,  for  they  are  alkaline  aluminum  silicates  which 
play  the  same  role  in  some  rare  igneous  rocks  that  the  feldspars 
do. 


SILICATES  355 

Leucite,  KAl(SiO3)2 

Form.  For  leucite  the  characteristic  form  is  well-defined 
embedded  crystals.  The  crystals  are  isometric;  the  only  com- 
mon form  is  the  trapezohedron  {211}  (Fig.  516).  Cross-sec- 
tions are  eight-sided  (Fig.  517). 

H.  =  5J^  to  6.  Sp.  gr.  2.5±. 

Color,  white  or  gray. 

Optical  Properties,  n  =  1.50.  Isotropic.  Fragments  are  ir- 
regular and  are  either  dark  between  crossed  nicols  or  have  very 
low  first-order  interference  colors. 

Chemical  Composition.  Potassium  aluminum  metasilicate 
KAl(Si03)2.  (K20  =  21.5  per  cent.).  A  little  sodium  is  sometimes 
present. 

Blowpipe  Tests.     Infusible. 

Decomposed  by  HC1  with  the 
separation  of  powdery  silica. 

Distinguishing  Features. 
Leucite  is  distinguished  by  its 
equidimensional  crystals,  which  are  FlG-  516'  FlG'  517' 

..  ,  .          J  ...  FIGS.  516-517.— Leucite. 

never  formed  in  cavities. 

Uses.  Rocks  containing  large  amounts  of  leucite  are  a  possible 
source  of  potassium  salts. 

Occurrence.  1.  In  certain  volcanic  rocks  in  which  leucite 
takes  the  place  of  the  feldspars  or  occurs  with  feldspars.  (Exceed- 
ingly rare  in  plutonic  rocks.)  Leucite  is  rare  in  the  United 
States,  but  occurs  in  large  quantities  in  the  Leucite  Hills, 
Wyoming. 

Nepheline,  (Na,K)AlSiO4-(NaAlSi3O8)a: 

Form.  Nepheline  occurs  in  embedded  crystals  or  grains  and 
in  massive  forms.  Crystals  are  hexagonal,  and  short  prismatic 
in  habit.  The  cross-sections  are  six-sided  and  rectangular. 

H.  =  5^  to  6.  Sp.gr.  2.6  +  . 

Color,  white,  gray,  or  reddish.     Luster,  greasy. 


356        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Optical  Properties.  n7(1.543).  -  w«(  1.538)  =  0.005.  Frag- 
ments are  irregular  with  low  first  order  interference  colors. 

Chemical  Composition.  Essentially  sodium  aluminum  ortho- 
silicate,  NaAlSiCX,  but  with^an  excess  of  silica  which  is  probably 
present  as  NaAlSiaOs.  Potassium  replaces  part  of  the  sodium. 

Blowpipe  Tests.     Fuses  at  4  to  a  colorless  glass. 

Gelatinizes  with  HC1. 

Distinguishing  Features.  Nepheline  resembles  quartz  but  is 
distinguished  by  its  inferior  hardness.  It  lacks  the  cleavage  of 
the  feldspars. 

Occurrence.  1.  In  nepheline  syenites,  phonolites,  and  other 
rare  soda-rich  igneous  rocks.  It  is  often  associated  with  ortho- 
clase,  but  never  with  quartz.  Magnet  Cove,  Arkansas. 


Sodalite, 

Form.  Sodalite  usually  occurs  in  disseminated  or  massive 
forms,  but  isometric  dodecahedral  crystals  are  sometimes  found. 

Cleavage,  indistinct  dodecahedral. 

H.  =  5^  to  6.  Sp.  gr.  2.3  ±. 

Color,  blue,  gray,  or  colorless. 

Optical  Properties,  n  =  1.483.  Isotropic.  Fragments  are 
irregular  and  colorless,  and  dark  between  crossed  nicols. 

Chemical  Composition.  Sodium  aluminum  chlorid  and  ortho- 
silicate,  Na4Al3Cl(Si04)3  or  3NaAlSi04'NaCl. 

Blowpipe  Tests.  Fusible  with  intumescence  to  a  colorless 
glass.  The  NaPO3  bead  with  CuO  gives  an  azure  blue  flame. 

Gelatinizes  with  HC1. 

Distinguishing  Features.  Blue  sodalite  is  distinguished  from 
lazurite  by  the  absence  of  associated  pyrite.  Unless  it  shows  the 
blue  color,  it  is  a  difficult  mineral  to  recognize.  The  chlorin  test 
in  a  NaPO3  bead  will  distinguish  it  from  most  other  minerals. 

Uses.  Some  varieties  of  sodalite  rocks  are  used  for  ornamental 
stones. 

Occurrence.  1.  In  soda-rich  igneous  rocks  such  as  nepheline 
syenites  and  phonolites. 


SILICATES  357 


Lazurite, 

Form.  Lazurite  usually  occurs  in  compact  massive  form,  more 
or  less  mixed  with  calcite,  pyrite,  and  other  silicates.  This  mix- 
ture is  known  as  lapis  lazuli. 

H.  =  5to5M-  .Sp.  gf.  2.4  ±. 

Color,  deep  blue.     Streak,  pale  blue. 

Optical  Properties,  n  about  1.50.  Isotropic.  Fragments  are 
irregular,  deep  blue,  non-pleochroic,  and  dark  between  crossed 
nicols. 

Chemical  Composition.  Sodium  aluminum  sulfid  and  ortho- 
silicate,  Na5Al  38(8104)3  or  3NaAlSiO4-Na2S.  It  usually  contains 
calcium  and  the  sulfate  radical,  both  due  to  isomorphous  re- 
placement. 

Blowpipe  Tests.  Fuses  (at  3)  with  intumescence  to  a  white 
glass. 

Soluble  in  HC1  with  gelatinization  and  with  the  evolution  of 
H2S. 

Distinguishing  Features.  Lazurite,  or  more  properly  lapis- 
lazuli,  is  distinguished  by  its  blue  color  and  by  the  presence  of 
pyrite.  The  latter  also  distinguishes  it  from  imitation  stones. 

Uses.  Lapis  lazuli  is  a  valuable  ornamental  stone.  It  was 
the  "sapphire"  of  the  ancients.  The  paint  called  ultramarine 
was  formerly  lapis  lazuli,  but  it  is  now  made  artificially. 

Occurrence.  1.  As  a  contact  mineral  in  crystalline  limestones 
associated  with  diopside  and  other  silicates.  The  quarries  at 
Badakshan  in  Afghanistan,  the  principal  source  of  lapis  lazuli, 
are  the  oldest  known  mines  in  existence. 

PYROXENE  GROUP 

The  following  hiinerals  constitute  a  mineral  group,  though 
they  are  not  strictly  isomorphous;  for  enstatite  and  hypersthene 
are  orthorhombic  and  rhodonite  is  triclinic,  while  the  others  are 
monoclinic. 


358 


INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Typical  analyses  of  the  more  important  members  of  the  pyrox- 
ene group  are  given  in  the  following  tabulation. 

Analyses  of  Minerals  of  the  Pyroxene  Group 


1 

MgO 

FeO 

CaO 

AhOa 

Fe2O3 

SiO2 

Misc. 

Enstatite  

36.9 

3.2 

1.3 

58.0 

ign.  =  0.8 

Bronzite  

29.7 

10.1 

1.3 

58.0 

MnO  =  1.0 

Hypersthene  

21.3 

21.3 

3.1 

0.4 

51.4 

Diopside  

17.3 

1.9 

25.0 

0.5 

1.0 

54.8 

Diopside  

16.1 

5.0 

24.9 

1.5 

0.6 

52.8 

Diopside 

10  0 

12  3 

22   1 

2  0 

1  3 

51    1 

MnO  =  0.1;  ign.  =  0.3 

Diallage 

16  4 

8  4 

20  3 

3.8 

50  2 

Augite  

16.0 

4.1 

19.0 

9.8 

4.5 

46.9 

Augite  

7.6 

9.4 

12.3 

21.5 

3.8 

42.2 

Na2O  =  3.0 

Augite  

13.2 

4.3 

21.3 

8.2 

3.7 

46.5 

TiO2  =  2.8 

Enstatite,  MgSiO3 

Form.  Enstatite  usually  occurs  in  lamellar  or  fibrous-lamellar 
masses.  The  mineral  is  orthorhombic,  but  distinct  crystals  are 
very  rare. 

Cleavage,  distinct  in  several  directions. 

H.  =  5>^  to  6.  Sp.  gr.  3.3  ± . 

Color,  bronze,  gray,  or  brown.     Luster,  metalloidal. 

Optical  Properties,  n7(1.67)-  na(1.66)  =  0.01.  Fragments 
are  prismatic  with  parallel  extinction,  low  first  order  interference 
colors,  and  positive  elongation. 

Chemical  Composition.  Magnesium  metasilicate;  (MgO  = 
40.0  per  cent.).  Ferrous  iron  usually  replaces  part  of  the  mag- 
nesium. Ferriferous  enstatite  is  called  bronzite. 

Blowpipe    Tests.     Fusible  on  thin  edges  (at  6). 

Insoluble  in  acids. 

Distinguishing  Features.  Enstatite  can  usually  be  dis- 
tinguished by  its  peculiar  bronze-yellow,  but  non-metallic, 
appearance. 

Occurrence.  1.  In  basic  igneous  rocks  such  as  peridotites  and 
gabbros. 

2.  In    meteorites, 


SILICATES 


359 


Hypersthene,  (Fe,Mg)SiO3 

Form.  Hypersthene  usually  occurs  in  cleavable  masses  or 
is  disseminated  through  rock  masses. 

Cleavage,  good  cleavage  in  one  direction. 

H.  =  5J£.  .     Sp.  gr.  3.4  ±. 

Color,  dark  brown  or  greenish  brown. 

Optical  Properties.  w7(1.70)  -  wa(1.69)  =  0.01.  Fragments 
are  prismatic  with  parallel  extinction,  bright  interference  colors, 
and  positive  elongation.  Hypersthene  is  usually  pleochroic; 
it  changes  from  pink  to  green. 

Chemical  Composition.  Iron  and  magnesium  metasilicate, 
(Fe,Mg)Si03. 

Blowpipe  Tests.  Fusible  (at  5)  to  a  black  glass.  On  charcoal 
in  R.F.  it  becomes  magnetic. 

Soluble  with  difficulty  in  HC1. 

Distinguishing  Features.  Hypersthene  is  difficult  to  dis- 
tinguish without  optical  tests. 

Occurrence.  1.  In  basic  igneous  rocks,  especially  gabbros  and 
norites. 

PYROXENE 

On  account  of  the  difficulty  of  distinguishing  some  of  the 
members  of  the  pyroxene  group  they  are  often  grouped  under 


FIG.  518. — Cross-sections  of  pyroxene  (diopside  and  augite). 

the  name,  common  pyroxene  or  pyroxene  proper.  Pyroxene 
(in  this  sense)  is  a  silicate  of  aluminum,  iron,  calcium,  and  magne- 
sium, which  is  often  found  in  monoclinic  prismatic  crystals  (see 
Figs.  519-526)  with  the  typical  cross-sections  shown  in  Fig. 
518.  It  comprises  two  fairly  well-defined  minerals,  diopside 
and  augite. 


360        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Diopside,  Ca(Mg,Fe)(SiO3)2 

Form.  In  crystals,  granular  masses,  disseminated  through 
rocks,  but  rarely  fibrous  or  columnar. 

Monoclinic  system. — Prismatic  class,  d  :  b  :  6  =  1.092  :  1  : 
0.589;  ft  =  74°  10'L  Usual  forms:  a{  100},  6{010),  mjllO), 
c{001),  p{lll},  o{221),  AJ311},  d{101}.  Interfacial  angles: 
w»m(110  :  110)  =  92°  50';  pp(lll  :  ill)  =  48°  29';  oo(221  : 
221)  =  84°  11'. 

The  habit  is  usually  prismatic  in  the  direction  of  the  c-axis. 
Figs.  519  to  522  represent  typical  crystals.  The  cross-section  of 


FIG.  519. 


FIG.  520.  FIG.  521. 

FIGS.  519-522. — Diopside. 


crystals  is  characteristic;  it  is  usually  four-  or  eight-sided  as 
represented  in  Fig.  518. 

Cleavage,  imperfect  in  two  directions  at  angles  of  87°  10'  and 
92°  50'  (parallel  to  the  unit  prism  { 110) ).  There  is  often  parting 
parallel  to  {001}  which  is  more  prominent  than  the  cleavage. 
The  variety  diallage  has  well-defined  parting  parallel  to  jlOOj. 

H.  =  4  to  6.  Sp.gr.  3.2  + . 

Color,  white,  gray,  or  green. 

Optical  Properties.  n7(1.70)  -  na(1.67)  =  0.03.  Fragments 
are  prismatic  and  colorless  or  pale  green  with  bright  interference 
colors  and  an  extinction  angle  of  20°-30°.  A  thin  parting  flake 


SILICATES  361 

parallel  to  {001}  will  give  an  interference  figure  consisting  of  an 
axial  bar  with  concentric  rings.  Diallage  has  parallel  extinction 
and  positive  elongation. 

Chemical  Composition.  Calcium  magnesium-ferrous  meta- 
silicate  varying  from  CaMg(SiO3)2  to  CaFe(SiO3)2.  Small 
amounts  of  aluminum,  ferric  .iron,  and  manganese  may  also  be 
present. 

Blowpipe  Tests.     Fusible  at  4  to  a  colorless  or  pale  green  glass. 

Insoluble  in  acids. 

Distinguishing  Features.  Diopside  is  distinguished  by  its 
crystal  form  and  imperfect  prismatic-  cleavage.  It  is  usually 
distinguished  from  augite  by  basal  parting  and  lighter  color. 

Occurrence.  1.  In  crystalline  limestones  as  a  contact  mineral 
associated  with  garnet. 

2.  In  schists  and  other  metamorphic  rocks,  both  in  the  rock 
mass  and  in  seams. 

3.  In  gabbros  and  peridotites  (the  variety  diallage). 

Augite,  mCaMg(SiO3)2  +  n(Mg,Fe)(Al,Fe)2SiO6 

Form.  Augite  usually  occurs  in  embedded  crystals.  The 
crystals  are  monoclinic,  prismatic  class,  with  the  forms:  ajlOOj, 
6{010[,  mUlO}_,_s{Illj.  Interfacial  angles:  mm(110:ll0)  = 
92°  50',  3s(lll:lll)  =  59°  11'.  The  habit  is  usually  prismatic 
(Figs.  523-526)  and  either  square  or  octagonal  in  outline  (see  Fig. 
518).  Twins  with  a  {100}  as  twin-plane  are  common  (Fig.  526). 

Cleavage,  imperfect  in  two  directions  parallel  to  {110},  and  at 
angles  of  92°  60'  and  87°  10'. 

H.  =  5M-  Sp.  gr.  3.3  ±. 

Color,  dark  green  to  black. 

Optical  Properties.  n7(1.73)  -  n«(1.71)  =  0.02.  Fragments 
are  prismatic  with  bright  interference  colors  and  large  extinction 
angles  (25°  to  40°).  The  thin  fragments  are  only  slightly  pleo- 
chroic,  if  at  all.  This  usually  distinguishes  augite  from  horn- 
blende. 


362        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Chemical  Composition.  An  isomorphous  mixture  of  CaMg 
(SiO3)2  and  (Mg,Fe)(Al,Fe)2SiO6  in  varying  proportions.  The 
presence  of  aluminum  and  ferric  iron  distinguishes  augite  from 
diopside.  Sodium  and  titanium  are  sometimes  present. 

Blowpipe  Tests.     Fusible  at  4  to  a  black  glass. 

Insoluble  in  acids. 

Distinguishing  Features.  Augite  is  distinguished  from  horn- 
blende by  its  square  or  octagonal  cross-section  and  imperfect 
prismatic  cleavage.  It  is  darker  colored  than  diopside. 

Occurrence.  1.  In  basic  igneous  rocks,  especially  basalts  and 
diabases,  often  as  phenocrysts  like  Figs.  523-526.  Bohemia. 

2.  In  basaltic  tuffs. 


m 


6  « 


o  m 


FIG.  523. 


FIG.  526. 


FIG.  524.  FIG.  525. 

FIGS.  523-526. — Augite. 

Rhodonite,  MnSiO3 

Form.  Rhodonite  is  found  in  cleavable  and  compact  masses, 
and  occasionally  in  euhedral  crystals.  The  crystals  are  triclinic, 
but  similar  to  diopside  and  augite  in  angles. 

Cleavage,  in  two  directions  at  angles  of  92J^°  (parallel  to  110) 
and  also  an  additional  parting  parallel  to  (100);  the  angle 
(100:  110)  is  48°  33'. 

H.  =  5>^.  Sp.  gr.  3.6  ±. 

Color,  pink  or  red,  often  stained  black  by  manganese  oxids. 

Optical  Properties.  nY(1.74)  -  w«(1.72)  =  0.02.  Fragments 
are  prismatic  with  bright  interference  colors  and  large  extinction 
angles  (20  to  25°). 


SILICATES 


363 


Chemical  Composition.  Manganese  metasilicate,  MnSiO3. 
Calcium  is  usually  present  and  sometimes  iron. 

Blowpipe  Tests.     Fusible  at  3  to  a  dark  glass. 

Partially  soluble  in  HC1. 

Distinguishing  Features.  Rhodonite  is  distinguished  from 
orthoclase  and  microcline  by  its  high  specific  gravity  and  from 
rhodochrosite  by  its  greater  hardness. 

Uses.  Conpact  rhodonite  is  used  as  an  ornamental  stone, 
especially  in  Russia. 

Occurrence.  1 .  In  high-temperature  veins  with  garnet.  Broken 
Hill,  New  South  Wales. 

2.  In  crystalline  limestones  with  willemite,  franklinite,  and 
zincite.  Franklin  Furnace,  New  Jersey. 

AMPHIBOLE  GROUP 

The  amphibole  group  is  parallel  to  the  pyroxene  group,  but  the 
triclinic  members  are  so  rare  that  no  account  of  them  will  be 
given  here.  The  amphiboles  differ  from  the  pyroxenes  mainly 
in  the  prism  and  cleavage  angle,  which  is  56° (and  124°)  instead  of 
87°  (and  93°) .  For  many  of  the  pyroxenes  there  are  corresponding 
amphiboles,  but  they  cannot  be  regarded  as  dimorphous  minerals. 
For  example,  diopside  is  CaMg(SiO3)2,  while  the  corresponding 
tremolite  is  CaMg3(SiO3)4.  Tremolite  and  hornblende  contain  a 
small  amount  of  water  of  constitution,  while  diopside  and  augite, 
if  unaltered,  contain  none. 

Analyses  of  Amphiboles 


CaO 

MgO 

FeO 

AhOa 

Fe2O3 

Si02 

Ti02 

H2O 

NazO 

K20 

F 

Anthophyllite.  . 

0.2 

28.7 

10.4 

0.6 

58.0 

1.7 

Tremolite  

13.2 

24.1 

0.6 

1.8 

57.7 

0.1 

1.6 

0.5 

0.2 

0.4 

Tremolite  (acti- 

nolite)  

12.1 

21.2 

5.5 

1.2 

0.8 

56.3 

1.8 

0.2 

0.3 

0.1 

Hornblende  .... 

9.8 

12.6 

10.5 

8.3 

6.9 

43.8 

0.8 

0.6 

3.4 

1.3 

1.8 

Hornblende.  .  .  . 

11.5 

11.2 

14.3 

11.6 

2.7 

42.0 

1.5 

0.6 

2.5 

0.1 

0.8 

Hornblende  .... 

12.0 

14.2 

2.2 

17.6 

7.2 

39.9 

1.7 

0.4 

3.2 

0.2 

0.1 

364        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Anthophyllite,    (Mg,Fe)SiO3 

Form.  Anthophyllite  usually  occurs  in  lamellar,  fibrous,  or 
asbestiform  masses,  which  are  often  radiating.  Crystals  are 
orthorhombic  but  terminal  faces  have  never  been  found. 

Cleavage,  perfect  in  two  directions  at  angles  of  54°  and  126°. 

H.  =  5-6.  Sp.  gr.  =  3.1±. 

Color,  gray  to  brown. 

Optical  Properties.  WT(  1.657)  --  na(1.633)  =  0.025.  Frag- 
ments are  prismatic  with  parallel  extinction  and  positive  elonga- 
tion. The  interference  colors  range  up  to  low  second-order. 

Chemical  Composition.  Magnesium,  iron  metasilicate  (Mg,- 
Fe)  SiO3.  Aluminum  is  of  ten  present  and  in  one  variety  (gedrite) 
is  prominent.  Most  specimens  of  anthophyllite  contain  a 
little  water.  A  typical  analysis  is  given  on  page  363. 

Blowpipe  Tests.  Anthophyllite  is  fusible  on  thin  edges  to 
a  black  glass.  In  the  closed  tube  it  may  give  a  little  water. 

Insoluble  in  acids. 

Distinguishing  Features.  Anthophyllite  is  distinguished  from 
the  other  amphiboles  by  the  absence  of  calcium  and  by  the 
parallel  extinction  in  fragments. 

Uses.  Anthophyllite  in  its  finely  fibrous  form  is  one  of  the 
varieties  of  asbestos.  It  is  quarried  at  Sails  Mountain,  Georgia. 

Occurrence.  1.  A  typical  metamorphic  mineral  occurring  in 
schists  and  gneisses. 


Tremolite,  Ca(Mg,Fe)3(SiO3) 


Form.  Tremolite  occurs  in  long  prismatic  crystals  and  in 
columnar  and  fibrous  aggregates.  Crystals  are  monoclinic  with 
the  prism  j  110)  and  the  pinacoid  {OlOj ,  but  rarely  have  terminal 
faces.  The  axial  ratios  and  interfacial  angles  are  like  those  of 
hornblende.  Characteristic  cross-sections  are  shown  in  Fig.  527. 

Cleavage,  in  two  directions  at  angles  of  56°  and  124°  parallel  to 
{110}.  The  cleavage  is  more  perfect  than  that  of  diopside. 

H.  =  5%.  (Fibers  may  appear  to  be  lower).     Sp.  gr.  3.0 ±. 


SILICATES  365 

Color,  white,  gray,  or  green.  The  green  varieties  are  some- 
times called  actinolite. 

Optical  Properties.  n7(1.636)  -  wa(1.611)  =  0.025.  Frag- 
ments are  prismatic  or  acicular  with  bright  interference  colors, 
positive  elongation,  and  extinction  angles  of  10°  to  15°. 

Chemical  Composition.  Calcium  magnesium-iron  metasilicate 
Ca(Mg,Fe)3(SiO3)4.  Ferriferous  varieties  with  more  than  2  or 
3  per  cent,  of  FeO  are  called  actinolite.  Aluminum  and  ferric 
iron  are  very  low  and  this  is  the  principal  chemical  distinction 
between  these  minerals  and  hornblende. 

Blowpipe    Tests.     Fusible  at  4  to  a  glass. 

Insoluble  in  acids. 

Distinguishing  Features.  Tremolite  is  easily  recognized  in 
typical  specimens  by  its  characteristic  cleavage.  It  is  distin- 


l» 

in 
FIG.  527. — Cross-sections  of  tremolite  (inc.  actinolite)  and  hornblende. 

guished  from  hornblende  only  by  its  color,  and  optical  characters. 

Uses.     The  fibrous  tremolite  is  one  kind  of  asbestos. 

Occurrence.  1.  In  crystalline  dolomitic  limestones.  Lee, 
Massachusetts. 

2.  In  schists,  often  associated  with  talc.    St.  Lawrence  County, 
New  York. 

3.  As  a  hydrothermal  alteration  product  of  pyroxene.     This 
is  a  fibrous  variety  known  as  uralite  which  usually  contains  some 
iron  and  aluminum  and  thiis  grades  into  hornblende. 

HORNBLENDE,  wCa(Mg,Fe)3(SiO3)4+n(Al,Fe)(F,OH)SiO3 

Form.  Hornblende  occurs  in  well-defined  crystals,  in  cleav- 
ages, in  disseminated  crystals  and  grains,  and  in  bladed  aggre- 
gates. 

Monoclinic  system.     Prismatic  class.     Axial  ratio :  a  :  b  :  6  = 


366        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

0.551  :1  : 0.293;  ft  =  73°  58'.  Usual  forms:  wjllO),  6{_010), 
a{100},  r{011|,  p{101}.  Interfacial  angles:  mm(110  :  110)  = 
55°  49',  rr{011  :011}  =  31°  32',  rp{011  :  101}  =  34°  25'. 
Habit  short  to  long  prismatic,  usually  pseudohexagonal  or  rhom- 
bic in  cross-section.  (See  Fig.  527.)  The  common  type  of 
hornblende  crystal  is  that  of  Fig.  528. 

Cleavage,  perfect  in  two  directions  at  angles  of  56°  and  124°, 
parallel  to  {110}. 

H.  =  5>^.  Sp.  gr.  3.2  ±. 

Color,  dark  green,  or  dark  brown  to  black. 
Optical    Properties,     nY(1.653)  -  na(1.629)  =  0.024.     Frag- 
ments are  prismatic,  and  green  or  brown  in  color. 
The  extinction  angle  varies  from  5°  to  20°  and  the 
elongation  is  positive.     Pleochroism  is  a  marked 
feature  of  hornblende.     The  colors  vary  from  pale 
to  deep  green,  from  yellowish-green  to  bluish-green, 
from  brown  to  greenish-brown,  or  from  pale  to  deep 
^  brown.     By  the   pleochroism  and  the  extinction 

FJP-    ^,28;~     angle,  hornblende  may  easily  be  distinguished  from 

.Hornblende.  .  . 

augite,  which  it  often  greatly  resembles. 

Chemical  Composition.  A  complex  metasilicate  of  calcium, 
magnesium,  ferrous  iron,  aluminum,  and  ferric  iron  with  fluorin 
and  hydroxyl.  The  formula  given  above  was  established  by  Pen- 
field  and  Stanley  (see  analyses,  page  363). 

Blowpipe  Tests.  Fusible  at  4  to  a  black  glass.  In  the  closed 
tube  it  gives  a  little  water  at  a  high  temperature. 

Insoluble  in  acids. 

Distinguishing  Features.  Hornblende  is  distinguished  from 
pyroxene  by  its  six-sided  cross-section  and  by  its  perfect  pris- 
matic cleavage  of  56°.  Optical  tests  may  be  necessary  to  dis- 
tinguish hornblende  from  pyroxene. 

Occurrence.  1.  In  volcanic  igneous  rocks  such  as  andesites 
and  certain  basalts. 

2.  In  plutonic  igneous  rocks,  especially  granites,  syenites,  and 
diorites,  rarely  in  gabbros  and  peridotites. 


SILICATES 


367 


3.  In  diabases  and  gabbros  as  a  magmatic  alteration  product 
of  augite  and  other  pyroxenes. 

4.  In  schists  and  gneisses  often  forming  rock  masses,  horn- 
blende schists  and  amphibolites. 

Glaucophane,  NaAl(SiO3)2- (Fe,Mg)SiO3 

Form.  Glaucophane  occurs  in  small  disseminated  crystals 
and  in  fibrous  masses.  Crystals  are  prismatic  in  habit  with  { 1 00 } , 
{010},  and  {110},  but  are  rarely  terminated.  The  cross-section 


FIG.  529.  FIG.  530.  FIG.  531. 

FIGS.  529-531. — Pleochroism  of  glaucophane. 

as  seen  in  thin  rock  sections  is  pseudohexagonal  or  rhombic 
(Fig.  527). 

Cleavage,  parallel  to  { 110),  i.e.,  in  two  directions  at  angles  of 
56°  and  124°. 

H.       6to6M-  Sp.  gr.  3.1  ±. 

Color,  blue  to  blue-black. 

Optical  Properties.  ny(l.Q4)  -  w«(1.62)  =  0.02.  Fragments 
are  prismatic  and  blue  in  color  with  pleochroism  from  blue 
to  violet.  The  extinction  is  practically  parallel  and  the  elonga- 
tion positive. 

In  thin  sections  glaucophane  shows  the  pleochroism  illustrated 
by  Figs.  529-531. 

Chemical  Composition.  Sodium,  aluminum,  iron-magnesium 
metasilicate,  NaAl(SiO3)2-(Fe,Mg)SiO3.  Glaucophane  is  one  of 
the  group  known  as  soda  amphiboles. 


368         INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Blowpipe  Tests.  Easily  fusible  (at  ,3)  to  a  dark  glass;  gives  an 
intense  yellow  flame. 

Insoluble  in  acids. 

Distinguishing  Features.  The  blue  to  blue-black  color  and 
occurrence  in  schists  are  distinctive. 

Occurrence.  1.  In  schists  and  gneisses  often  constituting 
the  main  part  of  the  rock.  Glaucophane  schists  and  glauco- 
phane  gneisses  are  especially  abundant  in  the  Coast  Ranges  of 
California. 


Beryl,  Be3Al2(SiO3)6 

Form.     For  beryl  the  characteristic  forms  are  crystals  and 
columnar  masses.     Beryl  crystallizes  in  the  hexagonal  system, 


FIG.  532. 


FIG.  533.  FIG.  534. 

FIGS.  532-535.— Beryl. 


FIG.  535. 


and  is  one  of  the  few  examples  of  the  di hexagonal  bipyramidal 
class^  Axial  ratio:  6  =  0.498.  Usual  forms:  c{0001},  m{l()To), 
a{1120},  p{10ll},  s{1121}._Interfacial  angles:  cp(_0001  :_1011) 
=  29°  56^;  cs{0001  :  1121J  =  44°  56r;  ma(1010  :  1120)  = 
30°  0'.  The  habit  is  usually  prismatic  (Figs.  532-534),  but  some- 
times tabular  (Fig.  535).  Crystals  are  often  very  large. 

H.  =  7^to8.  Sp.gr.  2.7  ±. 

Color,  usually  various  tints  of  green,  but  sometimes  white, 
yellow,  pink,  or  blue. 


SILICATES  369 

Optical  Properties.  WT  (1.570)  -  na(1.564)  =  0.006.  Frag- 
ments are  irregular  with  low  first-order  interference  colors. 

Chemical  Composition.  Beryllium  aluminum  metasilicate, 
Be3Al2(SiO3)6.  The  alkalies,  sodium,  lithium,  and  caesium,  often 
partly  replace  beryllium. 

Blowpipe  Tests.  Fusible  on  thin  edges  (at  6).  Insoluble  in 
acids. 

Distinguishing  Features.  Beryl  is  usually  distinguished  by 
crystal  habit.  It  is  harder  than  apatite  and  not  as  heavy  as 
corundum. 

Uses.  The  deep  green  variety,  emerald,  is  a  valuable  gem. 
The  best  emeralds  are  found  at  Muzo  in  Colombia.  Sea- 
green  (aquamarine)  and  pink  varieties  are  also  used  as  gems. 

Occurrence.  1.  In  granite  pegmatites  associated  with  topaz, 
albite,  lepidolite,  spodumene,  etc.  San  Diego  county,  California, 

2.  In  mica  schists  and  gneisses.     North  Carolina. 

3.  In  calcite  veins  in  limestone.     Muzo,  Colombia. 

Wollastonite,  CaSiO3 

Form.  Wollastonite  is  found  in  cleavable,  columnar,  fibrous, 
and  compact  masses.  Euhedral  crystals,  which  are  rare,  are 
monoclinic  and  are  elongate  in  the  direction  of  the  6-axis. 

Cleavage,  in  two  directions  (001  and  100)  at  angles  of  84^°. 

H.  =  4^  to  5.  Sp.gr.  2.8  ±. 

Color,  white  or  gray. 

Optical  Properties.  w7(  1.633)  -  wa(1.621)  =  0.012.  Frag- 
ments are  acicular  with  bright  interference  colors,  parallel 
extinction,  and  positive  elongation. 

Chemical  Composition.     Calcium  metasilicate,  CaSiO3. 

Blowpipe  Tests.  Fuses  (at  4)  to  a  white  glass,  giving  a  yellow- 
ish-red flame. 

It  is  decomposed  by  HC1  with  the  separation  of  powdery 
silica  and  usually  effervesces  because  of  admixed  calcite. 

Distinguishing   Features.     Wollastonite    resembles    tremolite 

24 


370        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

but  differs  in  cleavage.  It  may  be  necessary  to  use  optical  tests 
to  distinguish  them. 

Occurrence.  1.  In  crystalline  limestones  at  the  contact  with 
igneous  rocks,  often  associated  with  garnet,  diopside,  etc.  Lewis 
county,  New  York. 

2.  In  calcareous  inclusions  in  volcanic  rocks. 

Spodumene,  LiAl(SiO3)2 

Form.  Spodumene  occurs  in  rough  monoclinic  crystals  and  in 
cleavable  masses.  The  habit  of  the  crystals  is  prismatic  and 
usually  tabular  parallel  to  {100}. 

Cleavage,  in  two  directions  at  angles  of  93°  and  87°.  There  is 
also  parting  parallel  to  { 100}  at  times,  which  causes  the  mineral 
to  break  into  plates. 

H.  =  6^.  Sp.  gr.  3.1  ±. 

Color,  white,  gray,  colorless,  lilac,  greenish. 

Optical  Properties.  n7(1.67)  -  na(1.65)  =  0.02.  Fragments 
are  prismatic  with  first-order  interference  colors  and  oblique 
extinction  of  20°  to  25°. 

Chemical  Composition.  Lithium  aluminum  metasilicate,  LiAl 
(Si03)2.  Sodium  often  replaces  part  of  the  lithium. 

Blowpipe  Tests.  Fuses  (at  3J^)  to  a  clear  glass,  giving  a  purple- 
red  flame. 

Insoluble  in  acids. 

Distinguishing  Features.  Distinguished  from  feldspars  by 
its  higher  specific  gravity,  and  from  tremolite  by  differences  in 
cleavage. 

Uses.  Spodumene  has  been  used  to  some  extent  as  a  source  of 
lithium  salts.  A  transparent  lilac  variety  called  kunzite  is  used 
as  a  gem  and  also  a  transparent  emerald-green  variety  known  as 
hiddenite. 

Occurrence.  1.  In  granite  pegmatites  associated  with  albite, 
lepidolite,  tourmaline,  etc.  Pennington  county,  South  Dakota. 
One  crystal  from  the  Etta  Mine  in  this  county  measured  14 
meters  in  length. 


SILICATES 


371 


GARNET  GROUP  (Ca,Mg,Mn,Fe)3(Al,Fe,Cr)2(SiO4) 


Grossularite, 

Almandite, 

Pyrope,  Mg3Al2(SiO4)3 

Andradite,       Ca3Fe2(SiO4)3 

Form.  Garnet  is  found  in  distinct  crystals,  which  are  usually 
embedded,  in  granular  or  compact  masses,  and  in  the  form  of 
sand. 

Garnet  crystallizes  in  the  hexoctahedral  class  of  the  isometric 
system.  The  usual  forms  are  the  dodecahedron  d{  110}  and  the 
trapezohedron  WJ211J.  The  hexoctahedron  {321[  is  sometimes 
found,  but  the  cube  and  octahedron  are  exceedingly  rare  forms 
for  garnet.  Figs.  536-539  illustrate  commonly  occurring  garnet 


FIG.  536. 


FIG.  537.  FIG.  538. 

FIGS.  536-539.— Garnet. 


FIG.  539. 


crystals  ranging  from  dodecahedral  habit  to  trapezohedral  habit. 
Interfacial  angles:  dd(110  :  101)  =_60°;  dddlO  :  HO)  =  90°,  nn 
(211  :  121)  =  33°33J£';nn(211  :  2ll)  =  48°  11^';  dn(110  :  211) 
=  30°. 

Cleavage.     Usually  absent,  but  some  varieties  show  parting. 

H.  -  7.  Sp.  gr.  varies  from  3.5  to  4.2. 

Color,  various  tints  and  shades  of  red,  brown,  yellow,  green, 
and  occasionally  black. 

Optical  Properties,  n  varies  from  1.74  to  1.88.  Iso tropic. 
Fragments  are  irregular,  colorless  or  pale  red,  and  dark  between 
crossed  nicols,  but  some  varieties  have  weak  double  refraction. 

Chemical  Composition.  The  general  formula  of  garnet  is 
(Ca,Mg,Mn,Fe)3(Al,Fe,Cr)2(Si04)3.  The  four  most  common 
minerals  of  the  group  are: 


372        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Grossularite,  Ca3Al2(SiO4)3 1 . 736  to  1 . 763 

Almandite  FesAMSiOOs 1 . 778  to  1 . 815 

Pyrope,  Mg3Al2(SiO4)3 1 .741  to  1 . 760 

Andradite,  Ca3Fe2(SiO4)3 1 . 857  to  1 . 887 


Sp.  gr. 
3.53  ± 
4.25  ± 
3.51  ± 
3.75  ± 


Two  other  rare  minerals  of  the  group  are  also  known,  viz.; 
spessartite,  Mn3Al2(SiO4)3,  and  uvarovite  CasC^SiO^a. 

It  is  rare  to  find  a  garnet  that  corresponds  exactly  to  any  one 
of  these  as  can  be  seen  from  the  following  analyses: 

Analyses  of  Garnets 


CaO 

MgO 

FeO 

MnO 

AhOa 

Fe2Os 

Cr2O3 

Si02 

Grossularite  
Pyrope  

33.9 
5  0 

1.7 
17  9 

8   1 

0  5 

20.2 
22  4 

4.9 
5  5 



39.5 
40  9 

Almandite  

1  .4 

3  6 

33.8 

1  .1 

22  7 

37  6 

Almandite  

2.4 

3.7 

29.5 

4.8 

19.2 

4.9 

35.9 

0  5 

5  7 

37  2 

20  9 

35  7 

Andradite 

31   5 

tr 

0  3 

2  2 

30  4 

35  3 

Uvarovite  

31.6 

1.5 

5.7 

2.0 

21.8 

36.9 

Blowpipe  Tests.     Fusible  from  3  to  4. 

Insoluble  before  fusion,  but  after  fusion  (alone,  not  with 
Na2CO3)  it  gelatinizes  with  HC1. 

Distinguishing  Features.  Garnet  is  recognized  by  crystal 
habit,  absence  of  cleavage,  rather  high  specific  gravity,  and  by  its 
hardness. 

Uses.  Garnet  is  used  as  an  abrasive,  especially  for  finishing 
wood  and  leather.  New  York  has  furnished  the  total  domestic 
supply.  Spain  has  also  produced  abrasive  garnet  from  alluvial 
deposits.  Some  varieties  are  used  for  gems. 

Occurrence.  1.  In  crystalline  limestones  especially  at  con- 
tacts, associated  with  wollastonite,  diopside,  vesuvianite,  etc. 
(grossularite  and  andradite). 

2.  In  schists  and  gneisses  (almandite). 

3.  In  eclogites  with  pyroxenes  or  amphiboles. 


SILICATES 


373 


4.  In  granites  and  granite  pegmatites  (almandite) . 

5.  In  peridotites  and  derived  serpentines  (pyrope). 

6.  In  nepheline-  and  leucite-bearing  lavas,  such  as  phonolites, 
etc.  (melanite  variety  of  andradite). 

7.  In  sands. 

OLIVINE  GROUP— ORTHORHOMBIC 

The  olivine  group  is  a  group  of  orthorhombic  orthosilicate 
minerals  with  the  general  formula  R2"Si04  in  which  R"  may 
be  Mg,  Fe,  or  Mn.  The  minerals  of  this  group  are:  forsterite, 
Mg2SiO4;  fayalite,  Fe2SiO4;  olivine,  (Mg,Fe)2SiO4;  tephroite, 
Mn2SiO4;  and  moriticellite,  CaMgSi04.  Olivine  is  an  isomor- 
phous  mixture  of  magnesium  and  ferrous  silicates,  and  monti- 
cellite,  a  double  salt.  Olivine  is  the  only  common  mineral  of  the 
group. 

The  following  analyses  show  the  range  in  composition  of  these 
minerals : 


MgO 

FeO 

CaO 

MnO 

Si02 

Misc. 

Forsterite  
Olivine  

54.4 
50.3 

1.5 

8.5 

0.8 

42.8 
41.2 

ign.  =  0.8 

Olivine.  . 

44.1 

17.5 

39.2 

Olivine  
Fayalite 

30.6 
2  1 

28.1 
65.5 

1.4 

1.2 

38.9 
32.4 

Tephroite  
Monticellite  

1.4 

22.0 

1.1 

5.6 

1.0 
34.9 

65  6 

30.2 
37.9 

ign.  =0.4;  ZnO=0.3 

OLIVINE,  (Mg,Fe)2SiO4 

Form.  For  olivine  the  characteristic  occurrences  are  granular 
masses  or  disseminated  crystals  and  grains.  Crystals  are  ortho- 
rhombic  and  are  usually  tabular  in  habit.  Figure  540  represents  a 
crystal  with  all  the  seven  type  forms  of  the  rhombic  bipyramidal 
class.  a{100|,  6{010),  c{001),  m{110),  d{101},  /c{021),  pflll}. 

H.  =  6>^  to  7.  Sp.  gr.  3.3  ±. 


374        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Color,  yellowish  green  to  bottle  green. 

Optical     Properties.     ny(  1.699)  -  na(1.662)  =  0.037.     Frag- 
ments are  irregular  and  colorless  with  bright  interference  colors. 
Chemical   Composition.     Magnesium   and  iron   orthosilicate 
(Mg,Fe)2SiO4,  (Feb  =  5.0  to  30.0  per  cent.). 
Blowpipe  Tests.     Infusible. 
Gelatinizes  with  HC1. 
Distinguishing  Features.     Olivine  is  usually   recognized   by 

c  its  bottle-green  color  and  absence  of  cleavage. 

/j  '~f\\  Uses.  Clear  transparent  olivine  is  used  as 
Zj  lP\  ^  a  gem  under  the  name  peridot. 

Occurrence.  1.  In  pericfotites  with  enstatite 
or  diallage.  The  olivine  is  usually  partially 
altered  to  serpentine.  An  igneous  rock  com- 
posed practically  of  olivine  alone  is  called 
dunite. 

2.  In  basalts,  olivine  diabases,  and  olivine 
gabbros  as  an  essential  constituent. 

3.  In  tuffs  and  volcanic  bombs. 

4.  In  meteorites.     Pallasite  is  a  meteorite  rock  with  olivine 
filling  the  cavities  in  a  spongy  mass  of  iron. 

Forsterite,  Mg2SiO4 

Form.  Forsterite  usually  occurs  in  disseminated  anhedral 
or  subhedral  crystals.  Euhedral,  orthorhombic  crystals  like 
olivine  in  form  and  angles  are  known,  but  are  rare. 

H.  =  6H-  Sp.  gr.  3.25  ±. 

Color,  gray  to  pale  green  or  yellow. 

Optical  Properties.  ny(  1.670)  -  na(1.635)  =  0.035.  (For  pure 
artificial  Mg2SiO4).  The  indices  of  refraction  of  the  natural 
mineral  are  a  little  higher  on  account  of  the  presence  of  Fe2SiO4). 
Fragments  are  irregular  with  bright  interference  colors. 

Chemical  Composition.  Magnesium  orthosilicate  Mg2SiO4 
(MgO  =  57.1,  Si02  =  42.9),  with  a  little  ferrous  iron  replacing 
the  magnesium.  With  increasing  iron  it  grades  into  olivine. 


SILICATES  375 

Blowpipe  Tests.     Infusible  before  the  blowpipe. 

Soluble  in  HC1  with  gelatinization. 

Distinguishing  Features.  It  is  distinguished  from  olivine  by 
lower  iron  content  and  by  lower  indices  of  refraction.  From 
other  magnesium  silicates  it  is  distinguished  by  the  absence  of 
water. 

Occurrence.  1.  In  crystalline  limestones  as  the  product  of 
dedolomitization.  A  common  associate  is  spinel.  Bolton,  Mass- 
achusetts. 

2.  In  contact-metamorphic  zones  with  magnetite.  Phillips- 
burg,  Montana. 


Willemite,  Zn2SiO4 

Form.  This  mineral  is  usually  crystalline  massive  or  granular 
massive.  Crystals  are  hexagonal  and  prismatic  in  habit  with 
the  hexagonal  prism  {1120},  and  the  rhombohedron  {10ll{. 

H.  =  5>i  Sp.gr.  4.1± . 

Color,  pale  red,  yellow  to  green. 

Optical  Properties.  n7(1.717)  -  na(1.693)  =  0.024.  Fragments 
are  irregular  with  bright  interference  colors. 

Chemical  Composition.  Zinc  orthosilicate,  Zn2Si04;  (Zn  = 
58.0  per  cent.).  Manganese  often  replaces  part  of  the  zinc. 

Blowpipe  Tests.  Fusible  (at  %)  with  difficulty.  With 
cobalt  nitrate  solution  on  charcoal  the  assay  turns  blue  and  the 
sublimate  on  the  coal,  green.  Willemite  is  distinguished  from 
calamine  by  the  absence  of  water  in  the  closed  tube. 

Gives  a  fine  jelly  when  the  HC1  solution  is  heated. 

Uses.     Willemite  is  a  source  of  zinc  white  and  also  of  spelter. 

Distinguishing  Features.  Willemite  is  often  distinguished 
by  its  association  with  franklinite  (black)  and  zincite  (red). 

Occurrence.  1.  In  crystalline  limestone  intimately  mixed 
with  franklinite  and  zincite.  It  perhaps  has  been  formed  by  the 
metamorphism  of  calamine  present  in  the  original  sedimentary 
limestone.  Franklin  Furnace,  New  Jersey. 


376        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


2.  In  the  oxidized  zone  of  zinc  deposits.  (Calamine,  however, 
is  much  more  common  in  the  oxidized  zone). 

CALAMINE,  Zn2(OH)2SiO3 

Form.  Calamine  occurs  as  drusy  crystalline  coatings,  more 
rarely  in  botryoidal  and  stalactitic  forms  with  a  spherulitic 
structure,  and  also  massive.  Crystals  are  orthorhombic,  pyra- 
midal class.  The  habit  is  usually  tabular  parallel  to  {010}  and 
the  two  ends  of  the  crystal  are  differently  terminated.  Figure  541 
represents  a  typical  crystal.  The  usual  forms  are:  c{001(, 
&{010),  m{110[,  i{031|,  s{101},  £{301},  0J121J. 
Interfacial  angles:  c«(001:  301)  =  61°  20>^';  cs(001: 
101)  =  31°  23'. 

Cleavage,  perfect  parallel  to  the  length  of  the 
crystal.   . 

H.  =  5.  Sp.  gr.  3.4  ±. 

Color,  colorless,  white,  and  pale  colors. 
Optical  Properties.  nT(1.64)  -  w«(1.61)  =  0.03. 
Fragments  are  irregular  or  prismatic  with  parallel 
extinction.  Crystals  have  parallel  extinction  and 
positive  elongation.  The  interference  colors  are 
bright. 

Chemical  Composition.     Basic  zinc  metasilicate, 
.  Zn2(OH)2Si03  or  ZnSiO3-Zn(OH)2;  (Zn  =  54.2  per 

cent.,  H2O  =  7.5  per  cent.).     The  common  impurities  are  iron 
and  aluminum. 

Blowpipe  Tests.  Fusible  on  the  edges  (at  5).  In  the  closed 
tube  decrepitates  and  gives  off  water.  Heated  with  cobalt 
nitrate  solution  on  charcoal,  the  assay  becomes  blue  and  the 
sublimate  on  the  coal,  green. 

Soluble  in  HC1,  giving  a  fine  jelly  on  partial  evaporation. 

Distinguishing    Features.     Calamine    resembles    smithsonite 

and  is  often  distinguished  from  it  by  the  sharp,  well-defined 

crystals,  and  by  the  cleavage  parallel  to  the  length  of  the  crystals. 

Uses.     Calamine  is  one  of  the  ores  of  zinc. 


FIG.    541.  — 
Calamine. 


SILICATES 


377 


Occurrence.  1.  A  mineral  characteristic  of  the  oxidized  zone, 
usually  derived  from  sphalerite  and  often  associated  with  smith- 
sonite.  Granby,  Newton  county,  Missouri,  is  a  prominent 
locality. 

Scapolite,  m(3CaAl2Si2O8  CaCO3)  +  n(3NaAlSi3O8-NaCl). 

Form.  Scapolite  occurs  in  rough  crystals,  in  cleavable, 
columnar,  and  massive  forms.  Crystals  are  tetragonal  (tetra- 
gonal bi pyramidal  class),  prismatic  in  habit,  and  often  resemble 
diopside  crystals.  The  usual  forms  are: 
ajlOOj,  ra{110},  r{lll},  and  {101}.  In- 
terfacial  angles  :  rr(lll  :Tll)  =  43°  45', 
rar(110:lll)  =58°  12';  am(100: 110)  =  45°. 
Figure  542  represents  a  typical  scapolite 
crystal. 

Cleavage,  imperfect  parallel  to  {100}  and 
{110},  so  in  four  directions  in  one  zone  at 
angles  of  45°. 

H.  =  5^.  Sp.  gr.  2.56  to  2.77. 

Color,  white,  gray,  greenish,  or  reddish. 

Optical  Properties.  n7(1.595  to  1.550)  -  na(1.557  to  1.542)  = 
0.038  to  0.018.  Fragments  are  prismatic  with  parallel  extinction, 
bright  interference  colors,  and  negative  elongation. 

Chemical  Composition.  An  isomorphous  mixture  of  calcium 
aluminum  carbonate-silicate  with  sodium  aluminum  chlorid- 
silicate  in  varying  proportions. 

Blowpipe  Tests.  Easily  fusible  (at  3)  to  a  white  glass  with 
intumescence  coloring  the  flame  yellow. 

Partially  decomposed  by  HC1.  Some  varieties  give  slight 
effervescence  when  acid  is  hot. 

Distinguishing  Features.  Scapolite  resembles  diopside  and  the 
feldspars  but  is  distinguished  by  its  tetragonal  crystals  and  by 
optical  tests. 

Occurrence.  1.  In  crystalline  limestones  at  the  contacts  with 
igneous  rocks  and  associated  with  diopside,  garnet,  and  other 
silicates. 


FIG.  542. — Scapolite. 


378        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

2.  In  gabbros  along  the  border  of  apatite  veins.  Formed 
from  plagioclase  by  the  pneumatolytic  process  known  as  scapo- 
litization. 

Vesuvianite,  Ca6Al3(OH,F)(SiO4)5 

Form.  For  vesuvianite  the  characteristic  form  is  striated 
columnar  masses,  but  crystals  belonging  to  the  tetragonal 
system  are  also  common.  6  =  0.537.  The  usual  forms  are 
w{110},  a{100),  c{001},  p{lll},  Z{331[,  s{311}.  The  inter- 
facial  angles  are:  cp(001  :  111)  =  37°  13^';  op(100  :  111)  = 
64°  40^';  pp(lll:lTl)  =  50°  39';  mt(110  :  331)  =  23°  41J£; 
asClOO  :  311)  =  35°  10';  aw(100  :  110)  =  45°  0'.  The  habit  is 


FIG.  543. 


FIG.  544.  FIG.  545. 

FIGS.  543-546. — Vesuvianite. 


FIG.  546. 


prismatic  or  low  pyramidal  and  the  cross-section  is  usually  square 
or  octagonal.  Figs.  543-546  are  drawings  of  typical  vesuvianite 
crystals. 

H.  =  6J£.  Sp.  gr.  3.4  +  . 

Color,  yellow,  brown,  or  green. 

Optical  Properties.  ny(  1.723)  -  n«(1.722)  =  0.001.  Frag- 
ments are  irregular  with  anomalous  interference  colors. 

The  weak  double  refraction  may  be  proved  by  means  of  a 
gypsum  plate. 

Chemical  Composition.  Basic  calcium  aluminum  orthosili- 
cate,  probably  Ca6Al3(OH,F)(SiO4)5.  Iron  replaces  part  of 
the  aluminum  and  magnesium,  part  of  the  calcium.  Boron  is 
found  in  some  varieties. 

Blowpipe  Tests.     Fuses  (at  3)  with  intumescence  to  a  colored 


SILICATES  379 

glass.  In  the  closed  tube  at  a  high  temperature  it  gives  a  little 
water  (about  2  per  cent.). 

Slightly  decomposed  by  HC1.  After  fusion  (alone,  not  with 
Na2CO3)  it  gelatinizes  with  HC1. 

Distinguishing  Features.  Vesuvianite  is  distinguished  from 
most  other  minerals  by  its  crystal  form.  In  massive  specimens 
the  weak  double  refraction  with  peculiar  interference  colors  is 
the  best  test  to  apply. 

Uses.  Calif  ornite,  a  massive  jade-like  variety,  is  used  as  a 
semi-precious  stone.  It  occurs  in  Siskiyou  and  Fresno  counties, 
California. 

Occurrence.  1.  In  crystalline  limestones  at  the  contacts  with 
igneous  rocks,  and  associated  with  garnet,  diopside,  wollastonite, 
etc.  Crestmore,  California. 

2.  In  calcareous  inclusions  in  volcanic  rocks.     Vesuvius. 

Zircon,  ZrSiO4 

Form.  Zircon  is  practically  always  found  in  crystals  which  are 
either  embedded  or  occur  loose  in  sands. 

Zircon  is  one  of  the  best  examples  of  the  ditetragonal  bipyram- 
idal  class  of  the  tetragonal  system.  6  =  0.640.  Usual  forms: 
mfllOj,  _a{100),  pflll),  ^{331},  x{131}.  Interfacial  angles: 
pp(lll  :  111)  =  56°  40';  mpfllO  :  111)  =  47°  50';  ap(100  :  111) 
=  61°  40';  mu(UQ  :  331)  =  20°  12'.  The  habit  is  low  pyramidal 
or  prismatic  (Figs.  547  to  550). 

H.  =  1Y2.  Sp.gr.  4.7+ . 

Color,  usually  brown  but  also  red,  yellow,  and  colorless.  Lus- 
ter, adamantine. 

Optical  Properties.  w7(1.993)  -  n«(1.931)  =  0.062.  Frag- 
ments are  irregular  with  fourth-order  interference  colors. 

Chemical    Composition.     Zirconium    orthosilicate,    ZrSiO4. 

Blowpipe  Tests.     Infusible,  but  loses  color. 

Practically  insoluble  in  acids. 

Distinguishing  Features.  Zircon  is  easily  distinguished  by  its 
crystal  form  and  high  specific  gravity. 


380        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Uses.  Certain  kinds  of  zircon  called  hyacinth  are  used  as 
gems.  It  is  one  of  the  few  minerals  that  rivals  the  diamond  in 
brilliance  and  "fire. "  Zircon  is  the  source  of  zirconia  (ZrO2) 
used  as  a  glower  in  the  Nernst  lamp. 


P  I  P 


m 


m 


FIG.  547. 


FIG.  548. 
FIGS.  547-550.- 


FIG.  549. 
-Zircon. 


FIG.  550. 


Occurrence.  1.  As  an  accessory  mineral  in  igneous  rocks, 
especially  the  "acid"  rocks  rich  in  soda  such  as  syenites  and  soda 
granites.  El  Paso  county,  Colorado. 

2.  As  a  constituent  of  sands  and  gravels.     Ceylon. 

Topaz,  Al2(F,OH)2SiO4 

Form.  Topaz  occurs  in  well-defined  crystals  and  in  cleavable 
masses.  It  crystallizes  in  the  bipyramidal  class  of  the  ortho- 
rhombic  system,  d  :b  :  6  =  0.528  ::  0.477.  Usualforms:  m{  110) , 
Z(120},  c{001},/{021},  </[041},  w{lll},  o{221},  j{223}.  Inter- 
facial  angles:  rara(110  : 110)  =  55°_43';  11(120  :  120)  =  86°  49'; 
ml(UQ  :  120)  =  19°  44';  //(021  :  021)  =  87°  18';  yy(Ml  :  041) 
=  124°  42';  ci(001  :  223)  =  34°  14';  cw(001  :  111)  =45°  35'; 
co(001  :  221)  =  63°  54'.  The  habit  is  usually  prismatic,  with 
raj  110}  and  £{120}  about  equally  developed.  Figures  551-554 
represent  typical  crystals. 

Cleavage,  perfect  in  one  direction  parallel  to  { 001 } . 

H.  =  8.  Sp.gr.3.5±. 

Color,  colorless,  white,  yellowish,  bluish,  reddish. 

Optical    Properties.     n7(1.622)  -  na(1.613)  =  0.009.     Frag- 


SILICATES 


381 


ments  are  plates  with  irregular  outline  and  low  first  order  inter- 
ference colors.  Cleavage  flakes  give  a  positive  biaxial  inter- 
ference figure.  The  optical  orientation  of  topaz  is  a  =  a, 
0  =  b,  7  =  c;  hence  (010)  is  the  axial  plane. 

Chemical  Composition.  Aluminum  fluo-silicate,  A12(F,  OH)2- 
Si04.  Hydroxyl  replaces  part' of  the  fluorin. 

Blowpipe  Tests.  Infusible.  Heated  with  cobalt  nitrate 
solution  it  gives  a  deep  blue  color.  On  intense  ignition  in  a 
closed  tube  some  varieties  give  a  little  water.  On  heating  with 
NaPO3  in  the  closed  tube  it  etches  the  tube. 

Partially  decomposed  by  H2S04. 


\u  « 


in 


m 


FIG.  551. 


FIG.  552.  FIG.  553. 

FIGS.  551-554. — Topaz. 


FIG.  554. 


Distinguishing  Features.  Topaz  is  characterized  by  its 
perfect  cleavage  in  one  direction,  high  specific  gravity,  and  great 
hardness. 

Uses.  Topaz  is  sometimes  used  as  a  gem.  Many  of  the  stones 
that  pass  for  topaz  are  really  yellow  quartz  (citrine). 

Occurrence.  1.  In  granite  pegmatites  and  surrounding  rocks 
associated  with  tourmaline,  lepidolite,  albite,  fluorite,  apatite, 
beryl,  etc.  El  Paso  county,  Colorado. 

2.  In  cavities  in  rhyolites.     Thomas  Range,  Juab  county,  Utah. 

Andalusite,  Al2SiO5 

Form.  Andalusite  occurs  in  rough,  attached  or  embedded 
crystals,  in  columnar  masses,  and  in  rolled  pebbles.  The  crystals 


382        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


are  orthorhombic  and  prismatic  in  habit;  the  only  common  forms 
are  {110}  and  {001}  with  a  prism  angle  of  89°  12'  (110  :  HO). 
(Fig.  555). 

H.  =  7%.     (May  appear  softer  on  account  of  alteration.) 
Sp.  gr.  3.18  +  . 

Color,  usually  gray,  often  with  symmetrically  arranged  white 
or   black   areas.     This   variety  is   known   as   chiastolite.     (See 
Fig.  555.) 

Optical  Properties.  w7(1.64)  -  wa(1.63) 
=  0.01.  Fragments  are  irregular,  or 
prismatic  with  parallel  extinction,  and  color- 
less or  pleochroic  from  reddish  to  greenish. 
The  interference  colors  are  low  first-order. 
Chemical  Composition.  Aluminum  sili- 
cate, Al2SiO5,  or  Al203-SiO2,  with  the  same 
composition  as  sillimanite  and  kyariite. 

Blowpipe  Tests.  Infusible.  It  turns 
blue  when  heated  with  cobalt  nitrate 
solution. 

Insoluble  in  acids. 

Distinguishing  Features.  The  square 
prismatic  crystals  and  symmetrical  arranged 
inclusions  are  characteristic. 

Uses.  Some  varieties  have  been  used  as 
gems.  Chiastolite  is  sometimes  used  as  a 
watch  charm. 

Occurrence.     1.  In  schists  and  slates  as  a  product  of  contact  or 
regional  metamorphism.     It  is  usually  altered  to  a  soft,  fine- 
grained sericitic  aggregate.     Lancaster,  Massachusetts. 
2.  In  granite  pegmatites.     San  Diego  County,  California. 

Kyanite,  Al2SiO6 

Form.  For  kyanite  the  characteristic  form  is  bladed  crystals 
or  crystal  aggregates.  Crystals  are  triclinic,  tabular  parallel  to 
{ 100} ,  and  elongated  in  the  direction  of  the  c-axis. 


FIG.  555. — Andalusite 
(chiastolite) . 


SILICATES  383 

Cleavage,  perfect  in  one  direction  parallel  to  { 100} .  There  are 
also  imperfect  cleavages  in  other  directions. 

H.  =  4J<2  (parallel  to  the  length)  and  7  (perpendicular  to  the 
length).  Sp.  gr.  3.60 ±. 

Color,  blue,  bluish-gray,  green,  or  white,  often  colored  in  spots 
and  streaks. 

Optical  Properties.  ^(1.728)  -  na(1.712)  =  0.016.  Frag- 
ments are  prismatic  or  acicular  with  oblique  extinction  of  30°  and 
bright  interference  colors.  Cleavage  flakes  give  a  biaxial  inter- 
ference figure  with  the  axial  plane  oblique  to  the  edge. 

Chemical  Composition.  Aluminum  silicate,  Al2SiC>5  or  A12- 
03-SiO2.  Sillimanite,  andalusite,  and  kyanite  all  have  the 
same  composition,  but  differ  in  physical  properties. 

Blowpipe  Tests.  Infusible.  It  turns  blue  when  heated  with 
cobalt  nitrate  solution. 

Insoluble  in  acids. 

Distinguishing  Features.  The  bladed  structure  and  variation 
of  hardness  with  direction  are  characteristic. 

Occurrence.  1.  In  schists  and  gneisses  as  the  product  of 
regional  metamorphism.  It  is  often  associated  with  staurolite  and 
garnet.  Lincoln  county,  North  Carolina. 

Sillimanite,  Al2SiO5 

Form.  Sillimanite  occurs  in  prismatic  and  acicular  crystals 
and  in  fibrous  masses.  It  crystallizes  in  the  orthorhombic 
system,  but  distinct  crystals  are  rare.  The  prism  faces  with 
(110  :  110  =  88°)  are  usually  the  only  forms  present. 

Cleavage,  perfect  in  one  direction  parallel  to  {010}. 

H.  =  6K-     (Fibers  may  appear  to  be  lower).     Sp.  gr.  3.23 ±. 

Color,  brown,  gray,  or  white. 

Optical  Properties.  wT(1.681)  -  nttU.660)  =  0.021.  Frag- 
ments are  prismatic  or  acicular  with  parallel  extinction  and  posi- 
tive elongation.  The  interference  colors  are  bright. 

Chemical  Composition.  Aluminum  silicate,  Al2SiO5  or  A12- 
(VSiO2,  with  the  same  composition  as  andalusite  and  kyanite. 


384        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Blowpipe  Tests.  Infusible.  It  turns  blue  when  heated  with 
cobalt  nitrate  solution. 

Insoluble  in  acids. 

Distinguishing  Features.  Sillimanite  may  be  distinguished 
from  similar  minerals  by  optical  tests. 

Uses.  Jade-like  varieties  were  used  for  implements  by 
prehistoric  man  in  Europe.  The  artificial  mineral  is  now  a 
commercial  product. 

Occurrence.  1.  In  gneisses  and  schists,  as  the  result  of  meta- 
morphism.  Norwich,  Connecticut. 

EPIDOTE  GROUP— MONOCLINIC 

The  epidote  group  is  a  group  of  basic  orthosilicates  of  calcium 
and  aluminum,  with  essentially  the  general  formula  (Ca,Fe")2- 
(Al,Fe'/',Mn///,Ce,Cr//03(OH)^SiO4)3.  The  only  pure  compound 
is  the  mineral  clinozoisite  (fouqueite)  with  the  formula  Ca2- 
Al3(OH)(SiO4)3.  Epidote  is  an  isomorphous  mixture  of  this 
compound  with  Ca2Fe3( OH)  (8104)3,  a  possible  mineral  yet  to  be 
discovered.  Piedmontite  is  a  manganiferous  epidote  and 
allanite,  a  cerium-bearing  epidote. 

EPIDOTE,  Cas(Al,Fe)8(OH)(SiO4)3 

Form.  In  crystals,  in  columnar  aggregates,  and  in  granular 
masses.  The  crystals  are  monoclinic,  prismatic  in  habit,  and  are 
elongated  in  the  direction  of  the  6-axis  instead  of  the  c-axis,  as  in 
most  monoclinic  minerals.  Usual  forms:  a {100),  b {010},  cjOOlj, 
w{l20),  rjTOl},  rcjlll}.  Interfacial  angles:  ac(100:001)  =  64° 
37';  <y(001:101)  =  63°  42';  nw(Tll:TlT)  =  70°  29'.  cm(100: 
210)  =  35°29J^'.  Figure  556  is  an  end-view  of  a  typical  crystal, 
and  Fig.  557,  the  end-view  of  a  twin-crystal  with  a{100)  as  twin 
plane. 

Cleavage,  (001}  perfect;  {100}  imperfect. 

H.  =  6J^.  Sp.  gr.  3.4-3.5. 


SILICATES 


385 


Color,  usually  pistache-green,  but  varies  from  pale  yellowish- 
green  to  deep  brownish-green  and  almost  black,  according  to  the 
amount  of  iron  present. 

Optical  Properties.  n7(1.767)  -  wa(1.730)  =  0.037.  Frag- 
ments are  irregular  or  prismatic  with  parallel  extinction  and 
show  third-to  fourth-order  bright  interference  colors.  The  deep 
colored  varieties  are  pleochroic  from  colorless  to  pale  green. 
Cleavage  flakes  give  an  interference  figure  consisting  of  an  axial 
bar  with  concentric  rings. 

Chemical  Composition.  Basic  calcium  aluminum  and  iron 
orthosilicate,  Ca2(Al,Fe)3(OH)(SiO4)3,  an  isomorphous  mixture 


FIG.  557. 
FIGS.  556,  557. — Epidote. 

of  Ca2Al3(OH)(SiO4)3  (clinozoisite)  and  Ca2Fe3(OH)(Si04)3, 
unknown.  Ferrous  iron  may  replace  part  of  the  calcium.  The 
following  analyses  illustrate  the  isomorphism. 


CaO 

FeO 

AhOs 

Fe203 

Si02 

H20 

Clinozoisite  
Epidote  
Epidote 

24.4 
23.8 
23  9 

0.8 

0.5 

31.9 
29.5 
26.5 

3.0 

5.7 

8.2 

37.8 
38.0 
39.2 

2.2 
2.0 
22 

MnO  =  trace 
MnO  =0.2 

Epidote.  

23.3 

0.9 

22.6 

14.0 

37.8 

2.1 

Blowpipe  Tests.  Fusible  (at  3)  to  a  colored  glass  with  intumes- 
cence. In  the  closed  tube  at  a  high  temperature  it  gives  a  little 
water  (about  2  per  cent.). 

25 


386        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Partially  decomposed  by  HC1.  After  fusion  (alone,  not  with 
Na2CO3)  it  gelatinizes  with  HC1. 

Distinguishing  Features.  Epidote  can  usually  be  recognized 
by  its  peculiar  green  color.  From  clinozoisite  it  is  distinguished 
by  its  higher  indices  of  refraction,  higher  birefringence,  and  darker 
color. 

Occurrence.  1.  At  the  contact  between  igneous  rocks,  espe- 
cially granites  and  limestones  often  associated  with  copper 
minerals. 

2.  As  a  product  of  hydrothermal  alteration  in  various  kinds, 
of  rocks. 

3.  In  schists,  especially  as  the  product  of  intense  folding,  asso- 
ciated with  hornblende.     New  York  City. 

Clinozoisite  Ca2Al3(OH)(SiO4)3. 

Form.  In  monoclinic  crystals,  prismatic  in  the  direction  of 
the  6-axis,  and  in  columnar  and  granular  aggregates.  The  angles 
are  like  those  of  epidote  with  which  clinozoisite  is  isomorphous. 
It  is  dimorphous  with  orthorhombic  zoisite. 

Cleavage,  {0011  perfect;    {100J  imperfect. 

H.  =  6K.  Sp.  gr.  3.3-3.4. 

Color,  gray  to  green. 

Optical  Properties.  w7(1.72)  -  w«(1.71)  =  0.01.  Fragments 
are  irregular  or  prismatic  with  parallel  extinction.  Pleochroism 
not  noticeable  in  fragments  or  thin  sections.  The  interference 
colors  are  upper  first  or  lower  second  order  and  are  somewhat 
anomalous. 

Chemical  Composition.  Basic  calcium  aluminum  orthosilicate, 
Ca2Al3(OH)(SiO4)3,  (H2O  =  2.0).  A  little  ferric  iron  replaces  the 
aluminum.  Sometimes  ferrous  iron  is  present. 

Blowpipe  Tests.  Fusible  (at  3)  to  a  brown  glass  with  intum- 
escence. In  the  closed  tube  it  gives  a  little  water  at  a  high 
temperature. 

Partially  decomposed  by  HC1,  but  after  fusion  (alone)  it  gela- 
tinizes with  HC1. 


SILICATES  387 

Distinguishing  Features.  It  is  distinguished  from  epidote 
by  its  paler  color,  which  is  due  to  the  fact  that  it  contains  less 
iron. 

Occurrence.  1.  As  a  product  of  hydrothermal  alteration  in 
various  kinds  of  rocks.  It  is  sometimes  associated  with  ores. 


Prehnite, 

Form.  Prehnite  is  found  in  crystalline  druses  and  seams. 
Distinct  crystals  (orthorhombic)  are  very  rare.  The  imperfect 
crystals  are  usually  grouped  in  mammillary  and  globular  forms, 
showing  a  series  of  ridges. 

H.  =  6to6^.  Sp.  gr.  2.9  ±. 

Color,  usually  pale  green  or  white. 

Optical  Properties.  n7(1.649)  -  n«(1.616)  =  0.033.  Frag- 
ments are  irregular  with  bright  interference  colors. 

Chemical  Composition.  Acid  calcium  aluminum  orthosilicate, 
H2Ca2Al2(SiO4)3;  (H20  =  4.4  per  cent.). 

Blowpipe  Tests.  Easily  fusible  (at  2)  with  intumescence  to  a 
white  glass.  In  the  closed  tube  it  gives  water. 

Decomposed  by  HC1  and  after  fusion  (alone,  not  with  Na2COa) 
it  gelatinizes  with  HC1. 

Distinguishing  Features.  Prehnite  often  resembles  calamine, 
but  is  distinguished  by  its  lower  specific  gravity  and  by  its 
associates. 

Occurrence.  1.  A  secondary  mineral  in  cavities  of  diabases 
and  basalts  associated  with  datolite,  pectolite,  apophyllite, 
calcite,  and  the  zeolites.  Paterson,  New  Jersey. 

Staurolite,  FeAl5(OH)(SiO6)2 

Form.  Staurolite  crystallizes  in  the  orthorhombic  system  and 
is  rarely  found  massive.  The  habit  is  prismatic  with  the  forms 
c{001),  m{110),  6{010},  and  r{101}  (Fig.  558).  Interfacial 


388\       INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

angles  :  ww(110  :  lIO)  =50°  40':cr(001  :  101)  =55°  16'.     Cruci- 
form penetration  twins  with  {032}    as  twin-plane  are  common. 
H.  =  7  to  7M-  Sp.  gr.  3.7  ±. 

Color,  brown. 

Optical  Properties.  w7(  1.746)  -  wa  (1.736)  =  0.010.  Frag- 
ments are  irregular  with  upper  first-order  interference  colors  and 
are  pleochroic  from  light  to  deep  yellow. 

.  Chemical   Composition.     Basic   iron    aluminum 

silicate,  FeAl5(OH)(SiO6)2,  corresponding  to  the 
acid  H8SiO6(H4SiO4  +  2H2O).  The  ferrous  iron  is 
partly  replaced  by  magnesium,  and  the  aluminum 
by  ferric  iron. 

Distinguishing  Features.  The  crystal  form  of 
staurolite  is  distinctive  together  with  its  dark 
brown  color  and  its  occurrence  in  schists. 


FIG.    558. —          _,.          .        _  T    <•      -i  i 

Staurolite.  Blowpipe  Tests.       Infusible. 

Partially  decomposed  by  H2SO4. 

Occurrence.  1.  In  mica  schists  often  associated  with  kyanite, 
sillimanite,  and  garnet.  Fannin  county,  Georgia. 

TOURMALINE,  R9Al3B2(OH)2Si4O]9 

Form.  Tourmaline  usually  occurs  in  distinct,  attached  or 
embedded  crystals,  and  in  columnar  subradiating  aggregates. 
Tourmaline  is  the  type  example  of  the  ditrigonal  pyramidal  class 
of  the  hexagonal  system.  6  =  0.447.  Usual  fo^ms:  a{1120}, 
m{10TO},?7h{OnO},r{10Tlj,  c{ OlT2}_,_p{ 0221},  c^OOQl},  <} 2131}, 
w{3251),  a?{  1232},  _fi{ 0111},  ei{  1012},  _n{ 0001).  Interfacial 
angles  \rm(  1011  :  1010)  =  27°  20';  ewi(0112  :  0110)  =  75^30^'; 
<w»i(0221  :0110)  =  4=4°  3'j_ee(0ll2  :I012)  =  25°_2/;  rr(_10Tl  :Tl- 
01)  =  46°  52r;  oo(0221  :  2021)  =  77°  0';  xx(1232  : 1322)  =  21° 
18';_o:a:(1232  : 32l2)  =  43°  22^'j_  tt(2131  :  23ll)  =  63°  48'; 
tt(2131  :3121)  =  30°  38^',  am(1120  :  1010)  =  30°  Or;  aa(1120: 
2110)  =  60°  0'. 

The  habit  is  short  to  long  prismatic  and  the  cross-section 
three-,  six-,  or  nine-sided,  very  often  being  rounded  triangular 


SILICATES 


389 


like  a  spherical  triangle.  The  two  ends  of  the  crystals  are 
usually  terminated  differently.  Figures  559  to  562  represent 
typical  tourmaline  crystals. 

Cleavage,  none. 

H.  =  7  to  7%.  Sp.  gr.  3.00  to  3.25. 

Color,  usually  black,  but  also  brown,  green,  blue,  red,  and 
pink,  rarely  colorless.  The  exterior  and  interior  and  also  the 
opposite  ends  of  a  crystal  often  differ  in  color.  Transparent 
colored  crystals  show  pleochroism  with  a  dichroscope. 

Tourmaline  is  pyroelectric;  that  is,  a  crystal  which  has  been 
heated  will  on  cooling  develop  positive  electricity  at  one  end  and 
negative  electricity  at  the  other.  This  may  be  tested  with  a 
fine  hair. 


m 


FIG.  559. 


m 


FIG.  560.  FIG.  561. 

FIGS.  559-562. — Tourmaline. 


FIG.  562. 


Optical  Properties.  nT(1.653)  -  n«(1.631)  =  0.022.  Frag- 
ments are  irregular  or  prismatic  with  parallel  extinction.  The 
interference  colors  are  bright.  The  black  and  deep  colored 
varieties  are  pleochroic  (often  from  blue  to  smoke-colored), 
while  the  light  colored  varieties  are  colorless,  but  in  thick  frag- 
ments or  small  crystals  are  also  pleochroic. 

Chemical  Composition.  A  complex  borosilicate  of  aluminum, 
iron,  magnesium,  and  the  alkalies.  No  satisfactory  formula 
has  yet  been  established.  Penfield  gives  R91Al3B2(OH)2Si4Oi9, 
in  which  R1  is  iron,  magnesium,  and  the  alkalies.  The  following 
are  typical  analyses: 


390        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 
Analyses  of  Tourmaline 


Li2O 

Na2O 

H2O 

FeO 

MgO 

A12O3 

B2O3 

Si02 

Misc. 

1.  Pink  

1.9 

2.1 

3.4 

0.2 

42.2 

10.6 

37.6 

2.0 

2.  Pale  green  

1.3 

2.2 

3.3 

1.5 

41.3 

10.6 

36.7 

3.8 

3.  Brown  

tr 

0.9 

3.1 

0.9 

14.6 

28.5 

10.4 

35.3 

Ca  =  5.1 

4.  Black  

tr 

2  2 

3  6 

11\  9 

4   5 

31    1 

9  9 

34  9 

2  2 

5.  Black  

tr 

2.0 

3.6 

14:2 

1.0 

33.9 

9.6 

35.0 

0.6 

Three  principal  varieties  based  upon  composition  and  color 
are  recognized  viz.,  (1)  iron  tourmaline,  black  (analyses  4  and  5), 
(2)  magnesium  tourmaline,  brown  (analysis  3),  (3)  alkali  tour- 
maline, red,  green,  or  blue  (analyses  1  and  2). 

Blowpipe  Tests.  The  fusibility  varies  from  easy  fusibility  at 
3  (magnesium  variety)  to  infusibility  (alkali  variety).  In  the 
closed  tube  at  a  very  high  temperature  gives  water  (from  2  to  4 
per  cent.).  It  gives  a  momentary  green  flame  when  fused  with 
boric  acid  flux.  (3KHSO4  +  CaF2). 

Insoluble  in  acids.  After  fusion  (alone,  not  with  Na2CO3) 
it  gelatinizes  with  HC1. 

Distinguishing  Features.  The  triangular  cross-section  is 
characteristic  of  tourmaline.  The  black  or  common  variety  of 
tourmaline  is  distinguished  from  hornblende  by  the  cross-section 
and  by  the  absence  of  cleavage. 

Uses.  The  transparent  red  and  green  varieties  of  tourmaline 
are  used  as  gems. 

Occurrence.  1.  In  granite  pegmatites  often  associated  with 
albite,  lepidolite,  beryl,  apatite,  fluorite.  Pala,  San  Diego 
county,  California. 

2.  In  rocks  surrounding. granite  pegmatites  often  associated 
with  cassiterite,  topaz,  etc.     Greisen  is  a  quartz-muscovite  rock 
formed  from  granite  by  pneumatolysis;  luxullianite,  a  variety 
of  quartz  porphyry  in  which  the  quartz  is  partly  replaced  by 
tourmaline.     Cornwall,  England. 

3.  In  crystalline  limestones  (the  brown  magnesium  variety), 


SILICATES 


391 


associated  with  spinel,  phlogopite,  corundum,  etc.     Hamburg, 
New  Jersey. 

4.  In  high-temperature  veins  with  copper  and  lead  minerals. 
Meadow  Lake,  Nevada  county,  California. 

Datolite,  CaB(OH)SiO4 

Form.  Datolite  occurs  in  crystals,  crystalline  druses,  and 
crystalline  masses.  Crystals  are  monoclinic  and  are  usually  com- 
plex and  difficult  to  decipher.  As  the  angle  0  (the  acute  angle 
between  the  a  and  c-axes)  is  89°  51',  the  crystals  often  appear  to 
be  orthorhombic.  Figure  563  is 
an  orthographic  projection  of  a 
datolite  crystal  with  the  forms: 
raj_110),  a;j_102} ,  n{lll},  a{100(, 
€{Tl2},  X{113}  and/i{Tl4}. 

H.  =  5>i  Sp.gr.  2.9  + . 

Color,  colorless,  white,  pale 
green. 

Optical    Properties.     w7(  1.670) 
-  na(1.626)  =  0.044.     Fragments 
are  irregular  with  bright  interference  colors. 

Chemical  Composition.  Basic  calcium  boron  orthosilicate, 
CaB(OH)SiO4;  (H2O  =  5.6  per  cent.). 

Blowpipe  Tests.  Easily  fusible  (at  2)  with  intumescence  to  a 
glass,  coloring  the  flame  green.  In  the  closed  tube  it  gives  water. 

Gelatinizes  with  HC1. 

Distinguishing  Features.  Datolite  is  usually  recognized  by 
the  complex  crystals  without  cleavage  and  by  its  occurrence  in 
cavities  of  igneous  rocks  with  the  zeolites. 

Occurrence.  1.  Found  in  cavities  of  diabases  and  basalts 
associated  with  the  zeolites,  apophyllite,  prehnite,  pectolite 
[HNaCa8(SiO*)a],  etc. 

Axinite,  HCa2(Fe,Mn)Al2B(SiO4)4 

Form.  Axinite  occurs  in  crystals  and  crystalline  aggregates. 
The  crystals  furnish  one  of  the  best  examples  of  the  triclinic 


FIG.  563. —  Datolite. 


392        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


system.     The  habit  is  usually  tabular  and  the  cross-sections 
wedge-shaped.     Figure  564  represents  a  typical  crystal. 
H.  =  6>^  to  7.  Sp.  gr.  3.3  ±. 

Color,  violet,  brown,  smoky  gray. 

Optical  Properties.  n7(1.68)  -  wa(1.67)  =  0.01.  Fragments 
are  irregular  with  upper  first-order  interference  colors.  Thick 
fragments  are  pleochroic. 

Chemical  Composition.  An  acid  calcium  iron-manganese  alu- 
minum boron  silicate,  HCa2(Fe,Mn)Al2B(Si04)4. 

Blowpipe  Tests.  Easily  fusible  (at  2^) 
with  intumescence  to  a  dark  glass.  In  the 
closed  tube  at  a  high  temperature  it  gives 
a  little  water.  When  fused  with  KHS04 
and  CaF2  it  gives  a  green  flame. 

Insoluble  in  HC1.  Gelatinizes  after  fusion. 
Distinguishing    Features.      The    acute- 
angled  triclinic  crystals  are  characteristic. 
Occurrence.     1.  At   the   contact   of 
granites  with  basic  lime-rich  rocks  such  as 
schists  and  impure  limestones. 


FIG.  564. — Axinite. 


MICA  GROUP 

The  micas  are  acid  orthosilicates  of  aluminum  with  magnesium, 
ferrous  iron,  and  the  alkalies.  When  heated  to  a  high  tempera- 
ture they  give  from  2  to  5  per  cent,  of  water. 

The  micas  are  monoclinic,  but  pseudohexagonal  in  habit.  Dis- 
tinct terminated  crystals  are  very  rare.  The  very  perfect  cleav- 
age parallel  to  {001}  is  the  most  striking  feature  of  the  micas. 
They  are  optically  biaxial  with  varying  axial  angle.  In  the 
absence  of  crystal  faces,  cleavage  plates  of  the  micas  may  be  ori- 
ented by  means  of  the  percussion  figure  in  connection  with  the 
interference  figure.  A  sharp,  quick  blow  with  a  dull  conical 
point  develops  a  six-rayed  star.  If  the  interference  figure  lies 
along  one  of  the  rays  then  that  ray  is  the  direction  of  the  6-axis. 


SILICATES  393 

These  are  micas  of  the  first  class  and  are  represented  by  Fig.  565. 
If  the  interference  figure  lies  between  two  rays  of  the  percussion 
figure,  then  the  third  ray  is  the  direction  of  the  6-axis.  These 
are  micas  of  the  second  class  and  are  represented  by  Fig.  566. 
Muscovite  and  lepidolite  are  micas  of  the  first  class,  while  biotite 
and  phlogopite  are  micas  of  the  second  class. 


FIG.  565. — Mica  of  the  first  class.    FIG.  566. — Mica  of  the  second  class. 

MUSCOVITE,  H2KAl3(SiO4)3 

Form.  Muscovite  usually  occurs  in  cleavages  and  scaly 
masses  and  but  rarely  in  well-defined  crystals.  The  crystals  are 
tabular  in  habit,  and  pseudohexagonal  or  pseudorhombic,  but  are 
really  monoclinic.  Figure  567  represents  a  muscovite  crystal  in 
plan  and  side  elevation.  The  side  elevation  proves  it  to  be 
monoclinic. 

Cleavage,  very  perfect  in  one  direction  parallel  to  {001}. 

H.  =  2^.  Sp.gr.  2.83  ±. 

Color,  in  thick  sheets  various  tints  of  green,  yellow,  brown,  and 
red.  Thin  sheets  are  colorless  and  transparent. 

Optical  Properties.  n7(1.597)  -  wa  (1.560)  =  0.037.  Cleav- 
age flakes  give  low  first-order  interference  colors  (n$  —  na  = 
.004)  and  in  convergent  light,  a  fine  negative  biaxial  interference 
figure  with  large  axial  angle  (2E  =  60°  to  75°).  a  =  0;  6  =  7; 
c:a  =  1°. 

Chemical  Composition.  An  acid  potassium  aluminum  ortho- 
silicate,  H2KAl3(Si04)3;  (H2O  =  4.5  per  cent.).  The  potassium 
is  partially  replaced  by  sodium.  Some  varieties  contain  an 
excess  of  silica  over  that  required  by  the  formula. 


394        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


FIG.  567. — Muscovite. 


Blowpipe  Tests.  Fusible  on  thin  edges  (at  5)  and  whitens. 
In  the  closed  tube  it  gives  a  little  water. 

Insoluble  in  acids  and  not  decomposed  by  hot  concentrated 
H2S04. 

Distinguishing  Features.  Muscovite  is  easily  recognized  by  its 
elastic  cleavage,  and  flakes  with  perfect  cleavage  in  one  direction. 
Pink  muscovite  is  distinguished  from  lepidolite  by  the  easy 

fusibility  of  the  latter. 

Uses.  Muscovite  is  used  princi- 
pally as  an  insulator  for  electrical 
apparatus,  but  has  numerous  other 
uses.  India  is  the  principal  pro- 
ducer of  muscovite. 

Occurrence.  1.  In  granite  peg- 
matites and  granite  aplites. 

2.  In  schists  and  gneisses,  often 
the  main  constituent  of  the  mica  schists. 

3.  In  granites.     Granite  is  about  the  only  igneous  rock  in 
which  muscovite  occurs  as  an  original  constituent. 

4.  In  sandstones  and  sands  as  a  detrital  mineral. 

Sericite,   H2KAl3(SiO4)3 

Form.  Sericite  is  probably  dimorphous  with  muscovite.  It 
occurs  in  scaly  masses  and  very  rarely  is  distinct  crystals  which 
are  like  muscovite  in  form  and  habit  but  very  minute. 

Cleavage,  perhaps  in  one  direction. 

H.  =  1J£  to  2.  Sp.  gr.  2.8  ±. 

Color,  white,  gray,  or  pale  green. 

Optical  Properties.  The  optical  constants  are  near  those  for 
muscovite,  rc7(1.597)  —  na(1.560)  =  0.037.  Fragments  are 
cleavage  flakes  or  shreds  with  low  first-order  interference  colors. 
The  shreds  show  change  of  relief  when  examined  in  clove  oil 
under  the  microscope  in  polarized  light  (use  lower  Nicol  only). 

Chemical  Composition.  Acid  potassium  aluminum  orthosili- 
cate;  formula  probably  the  same  as  muscovite, 


SILICATES  395 

(H20  =  4.5  per  cent.).  The  potassium  is  partially  replaced  by 
sodium. 

Blowpipe  Tests.  Fusible  on  thin  edges  (at  5).  In  the  closed 
tube  it  yields  water. 

Insoluble  in  acids. 

Distinguishing  Features.  Sericite  is  recognized  by  its  pecu- 
liar silky  luster.  It  is  usually  called  talc  by  miners. 

Uses.  It  is  a  possible  source  of  potassium  salts.  Extensive 
deposits  of  sericite  schists  are  found  in  Georgia. 

Occurrence.  1.  As  a  product  of  hydro  thermal  alteration  in 
ore  deposits.  It  is,  according  to  the  researches  of  the  author,  a 
comparatively  late  mineral  formed  after  the  hypogene  sulfids. 

2.  In  schists  and  gneisses  forming  the  sericite  schists  and  sericite 
gneisses. 

Lepidolite,  LiKAl2(OH,F)(SiO3)3 

Form.  Lepidolite  usually  occurs  in  scaly  masses,  rarely  in 
crystals.  Crystals  are  pseudohexagonal  like  those  of  muscovite, 
but  are  much  smaller. 

Cleavage,  very  perfect  in  one  direction  (parallel  to  001). 

H.  =  2^  to  3>i  Sp.  gr.  2.84  ±. 

Color,  pale  to  deep  lilac. 

Optical  Properties.  nY(1.605)  -  n«(1.560)  =  0.045.  Cleav- 
age flakes  give  low  first-order  interference  colors,  and  in  conver- 
gent light  a  negative,  biaxial  interference  figure  with  large  axial 
angle  (2E  =  60°  to  80°). 

Chemical .  Composition.  Lithium,  potassium  aluminum  fluo- 
silicate,  LiKAl2^OH,F)(Si03)3. 

Blowpipe  Tests.  Easily  fusible  (at  2)  with  intumescence  to  a 
white  glass,  coloring  the  flame  purple.  In  the  closed  tube  on 
intense  ignition  it  gives  water  which  has  an  acid  reaction  due 
to  the  HF  formed. 

Partially  decomposed  by  HC1.  After  fusion  (alone,  not  with 
Na2CO3)  it  gelatinizes  with  HC1. 


396        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Distinguishing  Features.  Lepidolite  is  usually  recognized 
by  its  lilac  color  and  micaceous  character.  It  is  distinguished 
from  the  other  micas  by  its  easy  fusibility  and  by  the  lithium 
flame. 

Uses.    Lepidolite  is  a  source  of  lithium  salts. 

Occurrence.  1.  In  granite  pegmatites  and  surrounding  gran- 
ites associated  with  tourmaline,  albite,  muscovite,  spodumene, 
amblygonite,  etc.  Pala,  San  Diego  county,  California. 

BIOTITE,  (H,K)2(Mg,Fe)Al2(SiO4)3 

Form.  Biotite  occurs  in  embedded  crystals,  in  disseminated 
scales,  and  in  lamellar  masses.  Crystals  are  pseudohexagonal 
like  those  of  muscovite. 

Cleavage,   very  perfect  in  one  direction   (parallel  to    001). 

H.  =  2H  to  3.  Sp.  gr.  2.90  ±. 

Color,  black  or  dark  brown.     Thin  sheets  are  translucent. 

Optical  Properties.  ^(1.638)  -  wa(1.579)  =  0.044.  Cleavage 
flakes  are  almost  dark  between  crossed  nicols  and  in  convergent 
light  give  a  negative  biaxial  interference  figure  with  a  small  axial 
angle  (2E  =  0-20°)  which  is  sometimes  practically  uniaxial. 

Chemical  Composition.  Acid  potassium  magnesium  and  iron 
aluminum  orthoclase,  (H,K)2(Mg,Fe)2Al2(Si04)3.  Ferric  iron  re- 
places part  of  the  aluminum. 

Blowpipe  Tests.  Fusible  on  edges  (at  5)  and  turns  white. 
In  the  closed  tube  it  gives  a  little  water  (2  to  4  per  cent.)  on 
intense  ignition. 

Decomposed  by  hot  concentrated  H2SO4. 

Distinguishing  Features.  Biotite  is  distinguished  from  the 
other  micas  by  its  dark  color. 

Occurrence.  1.  In  many  kinds  of  igneous  rocks,  but  espe- 
cially prominent  in  granites,  and  also  in  certain  dike  rocks  known 
as  lamprophyres. 

2.  In  schists  and  gneisses,  sometimes  as  the  dominant  mineral, 
and  often  associated  with  muscovite. 

3.  In  high-temperature  veins. 


SILICATES  397 

Phlogopite,  H2KMg3Al(SiO4)3 

Form.  Like  the  other  micas,  phlogopite  occurs  in  crystals,  in 
disseminated  scales,  and  in  lamellar  masses.  The  crystals  are 
pseudohexagonal,  but  are  often  prismatic  in  habit  as  well  as 
tabular. 

Cleavage,  very  perfect  in  one  direction. 

H.  =  2K  to  3.  Sp.  gr.  2.80  ±. 

Color,  varies  from  pale  brown  to  dark  brown. 

Optical  Properties.  w7(1.606)  --  na(1.562)  =  0.044.  Cleav- 
age flakes  give  very  low  first-order  interference  colors  and,  in  con- 
vergent light,  a  negative  biaxial  interference  figure  with  a  small 
axial  angle  (2E  -  5°  to  20°). 

Chemical  Composition.  Acid  potassium  magnesium  aluminum 
orthosilicate,  H2KMg3Al(Si04)3.  It  also  contains  iron  and 
fluorin. 

Blowpipe  Tests.  Fusible  (at  5)  on  thin  edges  and  whitens.  In 
the  closed  tube  it  gives  a  little  water  on  intense  ignition. 

Easily  decomposed  by  concentrated  H2S04. 

Distinguishing  Features.  Phlogopite  is  darker  than  musco- 
vite  and  lighter  colored  than  biotite.  Its  occurrence  in  crystal- 
line limestone  is  characteristic. 

Uses.  Phlogopite  is  used  principally  as  an  insulator  in  elec- 
trical apparatus.  Canada  is  the  only  large  producer  of  phologo- 
pite. 

Occurrence.  1,  In  crystalline  limestones  associated  with 
spinel,  graphite,  chondrodite,  etc. 

2.  In  veins  with  apatite,  calcite,  and  diopside. 


CHLORITE,1  H8(Mg,Fe)5Al2(SiO6)3 

Form.  Chlorite  crystals  are  monoclinic  but  pseudohexagonal 
in  habit  and  resemble  crystals  of  the  micas.  The  mineral  also 
occurs  in  disseminated  flakes  and  in  scaly  masses. 

1  Chlorite  is  really  the  name  of  a  group  of  minerals,  but  on  account  of  the  difficulty  of 
distinguishing  them  they  are  all  included  under  one  heading. 


398        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Cleavage,  perfect  in  one  direction. 

H.  =  2  to  2>^.  Sp.  gr.  2.8  ±. 

Color,  green  of  various  tints  and  shades,  varying  from  almost 
white  to  almost  black. 

Cleavage  laminae  are  flexible,  but  not  elastic. 

Optical  Properties.  WT  (1.596)  -  w«(l.585)  =  0.011.  Frag- 
ments are  irregular,  green  in  color  with  faint  pleochroism,  and 
very  low  (often  Berlin  blue)  first-order  interference  colors. 
Cleavage  flakes  in  convergent  light  give  an  interference  figure 
which  is  either  uniaxial  or  biaxial  with  a  small  axial  angle  (2E 
=  0°-60°). 

Chemical  Composition.  Acid  magnesium  and  iron  aluminum 
silicate.  The  composition  varies  for  different  chlorites;  for 
one  of  them  (clinochlore)  the  formula  is  H8(Mg,Fe)5  Al2(Si06)3. 
In  some  varieties  chromium  and  ferric  iron  partly  replace  the 
aluminum. 

Blowpipe  Tests.  Fusible  with  difficulty  (at  5>£).  In  the 
closed  tube  gives  about  12  per  cent,  of  water  at  a  high 
temperature. 

Decomposed  by  H2S04. 

Distinguishing  Features.  Chlorite  is  characterized  by  its 
perfect  cleavage,  absence  of  elasticity  in  the  cleavage  flakes,  and 
green  color. 

Occurrence.  1.  A  secondary  mineral  in  igneous  rocks,  formed 
by  the  alteration  of  such  silicates  as  biotite,  hornblende,  augite, 
etc. 

2.  In  schists,  often  forming  independent  rock  masses,  the 
chlorite  schists.  These  are  formed  from  original  basic  igneous 
rocks. 

ANTIGORITE,  H4Mg3Si2O9 

Form.  Antigorite  has  never  been  found  in  crystals,  though  it 
often  occurs  pseudomorphous  after  other  crystallized  minerals. 
It  is  usually  compact  or  granular  massive,  but  also  occurs  in 
fibrous,  columnar,  and  lamellar  forms.  It  is  the  main  constituent 


SILICATES 


399 


of  serpentine,  which  is  properly  considered  a  rock  rather  than 
a  mineral. 

H.  =  3  to  4.  Sp.  gr.  2.5 ±. 

Color,  green  of  various  tints  and  shades,  from  greenish-white 
to  greenish-black.  It  is  also  often  yellow,  brown,  or  red,  and  the 
color  is  not  apt  to  be  uniform,  but  is  often  in  spots  and  streaks. 

Optical  Properties.  nY(1.571)  -  na(1.560)  =  0.011.  Frag- 
ments are  irregular,  or  prismatic  with  parallel  extinction  and 
positive  elongation.  The  interference  colors  are  low  first-order. 
The  irregular  fragments  show  aggregate  structure  between 
crossed  nicols. 

Chemical  Composition.  Acid  magnesium  silicate,  H4Mg3Si2O9 ; 
(H2O  =  12.9  per  cent.).  Part  of  the  magnesium  is  usually 
replaced  by  ferrous  iron.  Some  analyses  show  a  little  aluminum 
and  some  a  little  calcium.  These  constituents  are  largely  due 
to  residual  pyroxenes. 

Analyses  of  Antigorite 


MgO 

FeO 

CaO 

A1208 

Fe2O3 

SiO2 

H2O 

Misc. 

42.6 

0.1 

0.1 

0.3 

42.0 

14.7 

36.5 

1.9 

5.1 

42.9 

13.2 

NiO  =  0.6 

41.2 

2.4 

41.3 

14.5 

36.8 

7.2 

2.6 

41.6 

12.7 

Cr2O3  =  tr. 

Blowpipe  Tests.  In  the  closed  tube  it  gives  water  at  a  high 
temperature. 

Decomposed  by  HC1  with  the  separation  of  non-gelatinous 
silica. 

Distinguishing  Features.  Antigorite  is  distinguished  by  its 
green  color,  moderate  hardness  (4),  and  its  mottled,  veined,  or 
compact  massive  structure. 

Uses.     Serpentine  is  used  as  an  ornamental  stone. 

Occurrence.     1.  An  alteration  product  of  olivine  and  to  a  less 


400        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

extent  of  bronzite,  forming  the  metamorphic  rock  serpentine 
from  original  peridotite. 

2.  A  secondary  mineral  in  seams  and  veins  and  on  the  border 
of  serpentine  rocks. 

3.  An  alteration  product  of  diopside  and  forsterite  in  crystalline 
limestones;  these  form  the  rocks  known  as  ophicalcites. 

Chrysotile,  H4Mg3Si2O9 

Form.  Chrysotile  occurs  in  seams  and  always  has  a  fibrous 
structure. 

H.  =  2J£.  Sp.  gr.  2.2  +  . 

Color,  green  to  golden  yellow. 

Optical  Properties.  wT(1.55)  —  na(1.54)  =  0.01.  Fragments 
are  needles  with  parallel  extinction  and  positive  elongation. 

Chemical  Composition.  Chrysotile  is  dimorphous  with  anti- 
gorite  and  has  the  same  chemical  composition,  H4Mg3Si2O9. 
(H2O=  12.9  pfcr  cent.).  Ferrous  iron  may  replace  part  of  the 
magnesium. 

Blowpipe  Tests.     Fusible  (at  6)  in  minute  splinters. 

Decomposed  by  HC1  with  the  separation  of  fibers  of  silica. 

Distinguishing  Features.  Chrysotile  is  distinguished  from 
antigorite  by  its  fibrous  structure  and  by  the  difference  in  optical 
properties.  From  other  fibrous  minerals  it  is  usually  distin- 
guished by  its  association  in  serpentine  rocks.  The  parallel 
extinction  and  high  water  content  distinguish  it  from  tremolite 
asbestos. 

Uses.  Chrysotile  is  one  of  the  minerals  included  under  the 
term  abstestos,  which  is  so  extensively  used  as  a  refractory. 

Occurrence.  1.  In  seams  and  veins  in  serpentine  rocks, 
associated  with  antigorite.  Thetford,  Ontario. 

TALC,  H2Mg3(SiO3)4 

Form.  Talc  is  found  in  scales,  in  foliated,  compact,  or  fibrous 
masses.  Distinct  crystals  are  exceedingly  rare. 


SILICATES  401 

"*'  "  v 

Cleavage,  perfect  in  one  direction. 

H.  =  usually  1,  but  sometimes  4.  Sp.  gr.  2.7  +  . 

Color,  white,  gray,  or  pale  green.     Luster,  pearly. 

Optical  Properties.  w7(1.59)  -  n«(1.54)  =  0.05.  Cleavage 
flakes  give  a  negative  biaxial  interference  figure  with  a  small 
axial  angle  (2i!  =  10°-20°).  ' 

Chemical  Composition.  Acid  magnesium  metasilicate,  H2- 
Mg3(SiO3)4;  (H2O=  4.8  per  cent.).  It  usually  contains  iron  and 
aluminum  in  small  quantities. 

Blowpipe  Tests.  Fusible  (at  5J^)  on  thin  edges.  In  the 
closed  tube  gives  water  on  intense  ignition.  Heated  intensely 
with  cobalt  nitrate  solution,  white  varieties  give  a  faint  pink 
color. 

Not  decomposed  by  acids. 

Distinguishing  Features.  Talc  is  characterized  by  its  soapy 
feel.  It  can  be  distinguished  from  pyrophyllite  by  blowpipe  or 
chemical  tests. 

Uses.  Talc  is  used  for  soap,  French  chalk,  talcum  powder, 
and  in  the  form  of  soapstone  as  a  refractory  material.  A  fibrous 
variety  (agalite)  is  used  in  the  manufacture  of  paper.  The 
United  States  is  the  principal  producer  of  talc  and  soapstone. 
In  this  country  New  York,  Virginia,  and  Vermont  lead. 

Occurrence.  1.  A  secondary  mineral  occurring  as  an  altera- 
tion product  of  various  silicates  such  as  antigorite,  enstatite, 
and  tremolite  (including  actinolite). 

2.  In  schists  often  forming  the  rock  masses  known  as  talc 
schists  and  soapstones. 

Pyrophyllite,  HAl(SiO3)2 

Form.  In  radiated  forms  and  compact  masses,  but  not  in 
distinct  crystals. 

H.  =  \y^.  Sp.  gr.  2.8 ±. 

Color,  white,  yellow,  gray,  brown.     Luster,  pearly. 

Optical  Properties.     w7|1.59)  -  n«(1.57)  =  0.02.     Fragments 

26 


402        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

are  prismatic  with  parallel  extinction  and  positive  elongation. 
The  interference  colors  are  low  first-order. 

Chemical  Composition.  Acid  aluminum  metasilicate,  HA1- 
(SiO3)2;  (H2O  =  5.0  per  cent.). 

Blowpipe  Tests.  Infusible,  often  exfoliates.  In  the  closed 
tube  it  gives  water  on  intense  ignition.  Heated  with  cobalt 
nitrate  solution  it  becomes  deep  blue. 

Partially  decomposed  by  H2SO4. 

Distinguishing  Features.  Pyrophyllite  is  so  much  like  talc 
in  appearance  that  it  is  usually  necessary  to  make  the  cobalt 
nitrate  test  for  aluminum  in  order  to  prove  its  identity. 

Occurrence.     1.  In  schistose  metamorphic  rocks. 

2.  In  igneous  rocks  as  a  product  of  hydrotherrnal  alteration. 

Chondrodite,  Mg5(F,OH)2(SiO4)2 

Form.  Chondrodite  occurs  in  disseminated  crystals  and 
grains,  but  is  also  sometimes  massive.  Crystals  are  monoclinic, 
equidimensional,  and  rather  complex.  An  interesting  feature  of 
the  crystals  is  that  the  angle  0  (the  angle  between  the  a-  and  c- 
axes)  is  90°. 

H.  =  6  to  6K-  Sp.  gr.  3.20  ±. 

Color,  red,  orange,  yellow,  brown. 

Optical  Properties.  nY(1.639)  -  na (1.607)  =  0.032.  Frag- 
ments are  irregular  and  colorless,  or  yellow  with  slight  pleo- 
chroism.  The  interference  colors  are  bright. 

Chemical  Composition.  Basic  magnesium  orthosilicate,  Mg5- 
(F,OH)2(SiO4)2  or  Mg(F,OH)2-2Mg2SiO4.  Iron  replaces  part  of 
the  magnesium  and  hydroxyl  part  of  the  fluorin. 

Blowpipe  Tests.  Infusible.  In  the  closed  tube  it  gives  a  little 
water  (about  1.3  per  cent.)  at  a  high  temperature.  In  the 
closed  tube  with  NaPO3  it  etches  the  inside  of  the  tube. 

Gelatinizes  with  HC1. 

Distinguishing  Features.  Chondrodite  may  often  be  recog- 
nized by  its  characteristic  occurrence  in  crystalline  limestones. 


SILICATES 


403 


From  garnet  it  may  be  distinguished  by  its  low  specific  gravity 
and  by  optical  tests. 

Occurrence.  1.  In  crystalline  limestones  with  phlogopite, 
spinel,  etc.  Tilly  Foster  mine,  Brewster,  N.  Y. 

Kaolinite,  H4Al2Si2O9 

Form.  Kaolinite  is  sometimes  found  in  minute  pseudo-hexa- 
gonal (monoclinic)  crystals  of  tabular  habit.  Figure  568  repre- 
sents crystals  found  by  the  author  at  Argentine,  Kansas,  in  a 
dolomitic  limestone.  The  usual  occurrence  of  kaolinite  is  in 
clay-like  masses. 

H.  =  2to2>i  Sp.gr.  2.6+. 

Color,  white,  grayish,  yellowish,  etc. 
Luster,  pearly  to  dull. 

Optical  Properties.  wy(  1.567)  - 
na (1.561)  =  0.006.  Fragments  are  irregu- 
lar and  show  aggregate  structure  beween 
crossed  nicols. 

Chemical  Composition.  Acid  aluminum 
silicate,  H4Al2Si2O9;  (H2O  =  14.0  per  cent.). 
Iron  is  often  present  in  small  amounts  and 
thus  it  grades  into  nontronite  (H4Fe2Si209). 

Blowpipe  Tests.  Infusible  if  pure.  Heated  with  cobalt  nitrate 
solution  it  becomes  deep  blue.  In  the  closed  tube  it  gives  water. 

Insoluble  in  acids. 

Distinguishing  Features.  Soft  scaly  masses  of  minute  crystals 
with  pearly  luster  are  characteristic  of  kaolinite.  It  resembles 
halloysite,  sericite,  and  alunite  and  as  a  rule  can  only  be  dis- 
tinguished by  optical  tests. 

Uses.  Kaolin,  a  mixture  of  kaolinite  and  other  aluminum 
silicates  with  more  or  less  quartz,  feldspar,  etc.,  is  used  in  the 
manufacture  of  porcelain,  china,  and  pottery. 

Occurrence.  1.  A  secondary  mineral  formed  from  the  feld- 
spars and  other  silicates,  probably  by  the  action  of  meteoric 
water. 


FIG.  568. — Kaolinite 
crystals  (x  500). 


404        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Halloysite,    H4AlSi2O9(H2O)a! 

Form.  Halloysite  is  the  amorphous  equivalent  of  kaolinite. 
It  occurs  in  a  massive  form  and  occasionally  shows  colloform 
structure  in  cavities. 

H.  =  1  to  2.  Sp.  gr.  2.2  ±. 

Color,  white,  gray,  yellowish,  reddish,  etc. 

Optical  Properties,  n  =  1.55  +  .  Fragments  are  irregular 
and  dark  between  crossed  nicols. 

Chemical  Composition.  Acid  aluminum  silicate  with  a 
variable  amount  of  adsorbed  (or  dissolved)  water.  H4Al2Si2- 
O9(H2O)«.  (Total  water  =  15.0  to  25.0  per  cent.). 

Blowpipe  Tests.  Infusible  if  pure.  It  turns  blue  when 
ignited  with  cobalt  nitrate  solution.  Yields  abundant  water  in 
the  closed  tube. 

Decomposed  by  HC1. 

Distinguishing  Features.  Halloysite  is  distinguished  from 
kaolinite  by  optical  tests  and  by  its  higher  water  content. 

Uses.     Halloysite  is  the  principal  constituent  of  some  clays. 

Occurrence.  1.  In  sedimentary  beds.  Lawrence  county, 
Indiana. 

2.  In  veins  in  decomposed  igneous  rocks.  Stone  Mountain, 
Georgia. 

Garnierite,  H2(Ni,Mg)SiO4  H2O 

Form.  Garnierite  occurs  in  earthy  masses  and  has  never 
been  found  in  distinct  crystals,  though  the  polarizing  microscope 
proves  it  to  be  crystalline. 

H.  =  2  to  3.  Sp.gr.  2.5  ±. 

Color,  bright  green  to  pale  green. 

Optical  Properties,  n  about  1.59.  Fragments  are  irregular, 
greenish  in  color,  and  show  aggregate  structure  in  polarized 
light. 

Chemical  Composition.  A  hydrous  acid  nickel  and  magnesium 
orthosilicate,  H2(Ni,Mg)SiO4'H2O.  (Ni  =  10  to  35  per  cent.). 


SILICATES  405 

Blowpipe  Tests.  Infusible.  Heated  on  charcoal  it  becomes 
magnetic.  In  the  closed  tube  it  blackens  and  yields  water.  The 
borax  bead  is  violet  when  hot. 

Partially  decomposed  by  HC1. 

Distinguishing  Features.  Garnierite  is  usually  distinguished 
by  its  apple  green  color  and  earthy  appearance. 

Uses.  Next  to  pentlandite,  garnierite  is  the  chief  ore  of  nickel. 
The  French  colony  of  New  Caledonia  is  the  only  important 
locality. 

Occurrence.  1.  A  secondary  mineral  associated  with  serpen- 
tinized  peridotites  (it  is  probably  an  alteration  product  of  nickel- 
bearing  olivine).  Riddles,  Oregon. 

CHRYSOCOLLA,  CuSiO3  2H2O 

Form.  'Chrysocolla  is  a  cryptocrystalline  mineral  occurring 
in  seams  and  in  incrustations  which  sometimes  have  a  colloform 
surface.  Its  amorphous  equivalent  is  a  mineral  called  cornuite. 

H.  =  2  to  4.  Sp.  gr.  2.1  ±. 

Color,  bluish-green  or  greenish-blue. 

Optical  Properties.  nT(1.57)  -  na(1.46)  =  0.11.  Fragments 
are  irregular,  and  usually  show  aggregate  structure  in  polarized 
light. 

Chemical  Composition.  Hydrous  copper  metasilicate,  CuSiO3. 
2H2O;  Cu  =  36.1  per  cent.,  H2O  =  20.5  per  cent.). 

Blowpipe  Tests.  Infusible.  Colors  the  flame  green.  In  the 
closed  tube  it  blackens  and  gives  water. 

Decomposed  by  HC1  without  gelatinization. 

Distinguishing  Features.  Chrysocolla  resembles  turquois  but 
is  distinguished  from  it  by  its  inferior  hardness. 

Uses.  Chrysocolla  is  one  of  the  so-called  oxidized  ores  of 
copper. 

Occurrence.  1.  A  secondary  mineral  often  associated  with 
malachite,  azurite,  and  cuprite,  and  usually  found  in  the  upper 
workings  of  mines.  Gila  county,  Arizona. 


406         INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Glauconite,  KFe//r(SiO3)2  (H2O)X 

Form.  Glauconite  occurs  in  disseminated  grains  or  in  loosely 
cemented,  sandy  deposits  called  "greensand. "  The  grains  often 
have  the  form  of  foraminiferal  shells. 

H.  =  2.  Sp.  gr.  2.3  ±. 

Color,  green  to  dark  green. 

Optical  Properties.  ny  (1.628)  -  na  (1.610)  =  0.018  Frag- 
ments are  green  with  aggregate  polarization. 

Chemical  Composition.  Potassium  ferric  silicate  with  variable 
amounts  of  water;  probable  formula,  KFe///(Si03)2'(H2O)x;(K2O 
=  5  to  8  per  cent.).  Aluminum  replaces  part  of  the  iron.  Sod- 
ium, ferrous  iron,  and  magnesium  are  usually  present,  probably  as 
a  replacement  of  the  potassium. 

Blowpipe  Tests.  Easily  fusible  before  the  blowpipe  to  a 
black  magnetic  glass.  In  the  closed  tube  yields  water  (6  to 
10  per  cent.).  Practically  insoluble  in  HC1. 

Distinguishing  Features.  The  little  rounded  green  pellets 
are  characteristic.  It  resembles  some  varieties  of  chlorite  but 
has  higher  double  refraction. 

Uses.  As  a  fertilizer.  On  account  of  the  high  potash  con- 
tent it  may  in  the  future  be  used  for  potassium  salts. 

Occurrence.  1.  In  sandstones,  sands,  clays,  and  limestones. 
It  is  marine  in  origin  and  is  forming  in  the  ocean  at  a  depth  of 
about  100  fathoms.  Beds  of  "greensand  "  occur  in  the  Cretaceons 
of  New  Jersey. 

Apophyllite,  (H,K)2Ca(SiO3)2  H2O 

Form.  Usually  occurs  in  distinct  crystals  in  cavities  and 
along  seams.  Apophyllite  crystallizes  in  tetragonal  crystals  of 
varying  habit.  Usual  forms:  a{100(,  2/{310},  p{lll},  c{001|. 
Interfacial  angles:  cp(001  :  111)  =  60°  32';  ap(100  :  111)  =  52° 
0';  pp(lll  :  ill)  =  76°  0';  ay(10Q  :310)=  18°  26'.  (Figs.  569 
—572). 

Cleavage,  perfect  in  one  direction  parallel  to  { 001} . 

H.  =  4>ito5.  Sp.  gr.  2.3  ±. 


SILICATES 


407 


Color,  colorless  or  white.  Luster  of  (001)  face,  pearly;  of 
other  faces,  vitreous. 

Optical  Properties.  n7(1.535)  -  na(1.533)  =  0.002.  Frag- 
ments are  square  or  rectangular,  and  are  either  dark  between 
crossed  nicols  or  have  low  first-order  interference  colors.  Cleav- 
age flakes  give  a  positive  uniaxial  interference  figure  in  conver- 
gent light. 

Chemical  Composition.  A  hydrous  acid  calcium  metasilicate, 
(H,K)2Ca(SiO3)2'H2O.  A  little  potassium  replaces  part  of  the 
hydrogen  and  some  analyses  show  a  little  fluorin. 


FIG.  569. 


FIG.  570.  FIG.  571. 

FIGS.  569-572. — Apophyllite. 


FIG.  572. 


Blowpipe  Tests.  Easily  fusible  (at  2)  with  exfoliation  to  a 
white  enamel.  In  the  closed  tube  it  yields  water  (about  16  per 
cent.). 

Decomposed  by  HC1  with  the  separation  of  non-gelatinous 
silica. 

Distinguishing  Features.  The  tetragonal  crystals  with  per- 
fect basal  cleavage  are  highly  characteristic  of  apophyllite. 

Occurrence  1.  In  cavities  of  basic  igneous  rocks  associated 
with  zeolites.  West  Paterson,  New  Jersey. 


ZEOLITE  GROUP 

Under  the  zeolites  are  included  a  number  of  well  crystallized 
hydrous  silicates  of  aluminum  with  calcium  and  the  alkalies, 


408        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

which  are  chemically  similar  to  the  feldspars  except  for  the 
water  of  crystallization.  They  are  characterized  by  low  specific 
gravity  (2  to  2.5)  and  moderate  hardness  (3  to  5J^). 

They  are  all  decomposed  by  HC1  with  the  separation  of  slimy 
or  gelatinous  silica  and  are  easily  fusible  (at  2  to  3)  with  intumes- 
cence, hence  the  name  zeolite  from  the  Greek  word  meaning  to  boil. 

The  zeolites  are  usually  found  as  secondary  minerals  in  cavities 
of  such  basic  igneous  rocks  as  basalts  and  diabases.  Table 
Mt.  at  Golden,  Colorado,  and  Bergen  Hill,  New  Jersey,  are  promi- 
nent localities  for  zeolites. 

Heulandite,  H4CaAl2(SiO3)6  3H2O 

Form.     Heulandite    crystallizes    in    the    monoclinic    system. 
Usual  forms :b{ 010},  c{001(,  £{201},  s{201},ra{  110]_.     Angles:  ct- 
<.  (001:201)  =  63°  40';  cs(001:201)  =  66°  0'; 

6w(010:110)  =  68°  2'.  The  habit  is  usu- 
ally thick  tabular  parallel  to  {010}. 
The  unsymmetrical  outline  of  Fig.  573  is 
characteristic. 

Cleavage,  perfect  in  one  direction  parallel 
to  {010}. 

H.  =  3>^  to  4.  Sp.  gr.  2.2  ±. 

Color,   colorless,   white,   pale  brown, 
reddish.     Luster  pearly  on  the  (010)  face. 

Optical    Properties.     n7  (1.505)  - 

na(1.498)  =  0.007.  Fragments  are  plates  with  low  first-order 
interference  colors.  Cleavage  flakes  give  a  positive  biaxial  inter- 
ference figure  in  convergent  light. 

Chemical  Composition.  Hydrous  acid  calcium  metasilicate, 
H4CaAl2(SiO3)6-3H2O;  (H20  =  14.8  per  cent.).  The  calcium  is 
usually  partly  replaced  by  small  amounts  of  sodium,  potassium, 
and  strontium.  Brewsterite  is  a  similar  isomorphous  mineral 
with  the  calcium  largely  replaced  by  strontium  and  barium. 
Blowpipe  Tests.  Easily  fusible  (at  3)  with  exfoliation  to  a 
white  enamel.  In  the  closed  tube  it  gives  water. 


SILICATES 


409 


Decomposed  by  HC1  with  the  separation  of  non-gelatinous 

silica. 

Distinguishing  Features.  The  monoclinic  crystals  with  pearly 
luster  and  unsymmetrical  outlines  are  characteristic. 

Occurrence.  1.  A  secondary  mineral  in  cavities  of  basic 
igneous  rocks,  associated  with. other  zeolites. 

Stilbite,H4rCa,Na2)Al2(Si03)6-4H20 

Form.  Stilbite  usually  occurs  in  indistinct  crystals  or  in 
sheaf-like  aggregates.  Crystals  are  monoclinic  but  are  pseudo- 
orthorhombic  by  twinning.  The  symmetrical 
outline  of  Fig.  574  is  typical  of  stilbite. 

Cleavage,  in  one  direction  fairly  good. 

H.  =  3Mto4.  Sp.  gr.  2.1  ±. 

Color,  white,  yellow,  brown. 

Optical  Properties.  ny(  1.500)  -  ntt(1.494) 
=  0.006.  Fragments  are  prismatic  with 
parallel  extinction  and  negative  elongation. 
The  interference  colors  are  upper  first-order. 

Chemical  Composition.     Hydrous  acid 
calcium  and  sodium  aluminum  metasilicate, 
H4(Ca,Na2)Al2(SiO3)6-4H2O;    (H20  =  17.2     FIG.  574.— stilbite. 
per  cent,  if  Ca:Na  =  6:1). 

Blowpipe  Tests.  Easily  fusible  (at  3)  with  exfoliation  to  a 
white  enamel.  In  the  closed  tube  it  yields  water. 

Decomposed  by  HC1  with  the  separation  of  non-gelatinous 
silica. 

Distinguishing  Features.  The  symmetrical  outline  and  sheaf- 
like  grouping  of  the  crystals  is  characteristic.  The  cleavage  is  not 
as  perfect  as  that  of  heulandite. 

Occurrence.  1.  A  secondary  mineral  in  cavities  and  seams 
of  igneous  rocks,  especially  basalts  and  diabases. 

2.  In  miarolitic  cavities  of  granites  and  pegmatites  as  the  last 
mineral  to  be  formed. 


410        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Chabazite,  (Ca,Na2)Al2(SiO3)6-6H2O 

Form.     Chabazite  practically  always  occurs  in  distinct  cube- 
like  rhombohedral  crystals   (10Tl:Il01)  =  85°   14'    (Fig.  575). 
Penetration  twins  with  6  as  the  twin-axis  are  common. 
H.  =  4^.  Sp.  gr.  2.1  ±. 

Color,  white,  colorless,  pink,  red. 

Optical  Properties.  nTQ.488)  -  na(1.485)  =  0.003.  Frag- 
ments are  nearly  square  rhombs  or  are  irregular.  The  inter- 
ference colors  are  low  first-order. 

Chemical  Composition.  Hydrous 
calcium-sodium  aluminum  metasilicate, 
(Ca,Na2)Al2(SiO3)4-6H2O.  A  little  po- 
tassium is  usually  present. 

Blowpipe  Tests.  Fuses  (at  3)  with  in- 
tumescence to  a  white  glass.  In  the 
closed  tube  it  yields  water  (about  21  per 
cent.).  Decomposed  by  HC1  with  the 
separation  of  non-gelatinous  silica. 
FIG.  575.— Chabazite.  Distinguishing  Features.  The  cube- 

like  rhombohedral  crystals  are  charac- 
teristic. It  is  distinguished  from  calcite  by  the  absence  of  perfect 
cleavage  as  well  as  by  optical  tests. 

Occurrence.  1.  A  secondary  mineral  in  cavities  and  seams  of 
igneous  rocks  associated  with  the  other  zeolites. 

Analcite,   NaAl(SiO3)2  H2O 

Form.  Analcite  occurs  in  attached  crystals  or  in  druses 
lining  cavities  and  seams.  It  is  isometric  in  crystallization;  the 
only  common  form  is  the  trapezohedron,  {211},  the  same 
form  that  is  common  on  garnet  (Fig.  576). 

H.  =  5to5H-  Sp.gr.  2.25  ±. 

Color,  colorless  or  white. 

Optical  Properties,  n  =  1.487.  Isotropic.  Fragments  are 
irregular  and  dark  between  crossed  nicols. 


SILICATES 


411 


FIG.       576.— 
Analcite. 


Chemical  Composition.     Hydrous  sodium  aluminum  metasili- 
cate,  NaAl(SiO)3)2-H2O;  (H2O  =  8.2  per  cent.). 

Blowpipe  Tests.     Fusible  at  3^  to  a  colorless  glass.     In  the 
closed  tube  it  yields  water. 

Decomposed  by  HC1  with  the  separation  of 
gelatinous  silica. 

Distinguishing  Features.  Analcite  is  similar 
in  form  to  leucite  and  garnet;  from  these  it  is 
distinguished  by  its  mode  of  occurrence. 

Occurrence.  1.  As  secondary  mineral  in 
seams  and  cavities  of  basic  igneous  rocks. 

2.  As  Jate  magmatic  mineral  in  certain  diabases  and  basalts. 

Natrolite,  Na2Al2Si3Oi0-2H2O 

Form.  This  mineral  occurs  in  divergent  crystal  groups  or  in 
fibrous  masses.  Crystals  are  orthorhombic  but  apparently  tetra- 
gonal (110  :  HO  =  88°  45').  The  habit  is  long  pris- 
matic or  acicular,  terminated  by  the  low  bipyramid 
{111}.  Figure  577  represents  a  typical  natrolite 
crystal.  The  presence  of  the  side  pinacoid  b  proves 
it  to  be  orthorhombic. 

H.  =  5.  Sp.gr.  2.25  ±. 

Color,  colorless  or  white. 

Optical  Properties.  nT(1.488)-n«(1.475)  =  0.013. 
Fragments  are  prismatic  or  acicular  with  parallel  ex- 
tinction, positive  elongation,  and  bright  interference 
colors. 

Chemical  Composition.  Hydrous  sodium  alu- 
minum silicate,  Na2Al2Si3Oio'2H20;  (H2O  =  9.5  per 
cent.). 

Blowpipe  Tests.  Easily  fusible  (at  2J£)  to  a  color- 
less glass  giving  a  yellow  flame.  In  the  closed  tube 
it  yields  water. 

Decomposed    by    HC1    and    on    evaporation    the    solution 
gelatinizes. 


FIG.      577. — 
Natrolite. 


412        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Distinguishing  Features.  Natrolite  is  distinguished  from 
the  other  zeolites  and  from  aragonite  by  the  square  cross-sec- 
tion of  the  crystals. 

Occurrence.  1.  A  secondary  mineral  occurring  in  cavities  of 
basalts  and  diabases. 

Titanite,  CaTiSiO5 

Form.  Titanite  or  sphene  occurs  in  attached  crystals,  and  in 
disseminated  crystals  and  grains.  Crystals  are  monoclinic  of 
varied  habit,  and  are  usually  acute  rhombic  in  cross-section. 
The  envelope-shaped  form  of  Fig.  578  is  typical.  Usual  forms: 
c{001}^  rafllO},  rzjlll}.  Interfacial  angles:  mm 
Q10  :IlO)  =  66°  29';  nn  (111: ill)  =  43°  49';  cm 
(001:110)  =  65°  30';  en  (001:111)  =  38°  16'. 

Cleavage.     There  is  sometimes  prominent  parting 
in  two  directions  at  angles  of  54°. 

H.  =  5  to  5K-  Sp.  gr.  3.45  ± . 

Color,   varying  tints  and  shades  of   yellow  and 
brown.     Luster,  adamantine  or  subadamantine. 


FIG    578—       °Ptical    Properties.     nT^2.00)  -  n«(1.88)  =  0.12. 

Titanite.  Fragments  are  irregular  and  slightly  pleochroic  with 
very  high-order  interference  colors. 

Chemical  Composition.  Calcium  titano-silicate,  CaTiSiO5,  a 
salt  of  H2Si2O5  in  which  one  atom  of  silicon  is  replaced  by  one  of 
titanium.  Iron  is  usually  present  in  small  amounts. 

Blowpipe  Tests.  Fusible  (at  4)  to  a  colored  glass.  It  gives 
a  violet  NaPO3  bead  in  R.F. 

Partially  soluble  in  HC1. 

Distinguishing  Features.  Titanite  is  often  distinguished  by 
the  acute-angled  crystals  which  are  envelope-shaped. 

Occurrence.  1.  A  common  and  widely  distributed  accessory 
constituent  of  plutonic  igneous  rocks  (granites,  granodiorites, 
dioriteSj  syenites). 

2.  In  clefts  and  seams  or  disseminated  through  metamorphic 
rocks,  probably  often  formed  from  titaniferous  pyroxenes. 


SILICATES 


413 


MINERALOIDS 

A  mineral  may  be  defined  as  a  naturally  occurring,  homo- 
geneous, inorganic  substance  of  definite  or  fairly  definite  chemical 
composition.  Now  there  are  some  homogeneous  substances 
found  in  the  earth's  outer  shell  which  do  not  fulfill  the  conditions 
of  the  above  definition,  yet  they  deserve  the  attention  of 
the  student  of  mineralogy.  Among  these  substances  are  the 
glasses  and  the  hydrocarbons.  The  glasses  are  inorganic  but 
are  too  indefinite  in  chemical  composition  to"  be  considered 
minerals,  while  the  hydrocarbons,  though  occasionally  of  de- 
finite composition,  are  organic.  These  two  classes  of  substances 
may  be  treated  in  an  appendix  under  the  term  mineraloid. 

Glass 

Form.  Amorphous,  usually  massive  and  structureless,  but 
sometimes  vesicular,  spheroidal  (perlitic),  or  banded. 

H.  =  5  to  7.  Sp.  gr.  =  2.2  to  2.7. 

Color,  colorless,  gray,  black;  sometimes  green,  brown,  or  red. 
Translucent  to  transparent  on  thin  edges. 

Luster.     Vitreous  to  resinous,  dull  if  devitrified. 

Optical  Properties.  n=  1.48  to  1.67  (increases  in  general  with 
decrease  of  SiO2).  Usually  isotropic  but  may  be  doubly  refract- 
ing due  to  strain,  especially  the  perlitic  varieties. 

Chemical  Composition.  Variable,  contains  SiO2,  A12O3,  Fe2O3, 
FeO,  MgO,  CaO,  Na20,  and  K20  in  amounts  comparable  to 

Table  of  Analyses  of  Natural  Glass 


0 
02 

q 

< 

6 

A 

3 
fe 

% 
S 

3 

o 

q 

a 
fc 

q 
C? 

O 

w 

Misc. 

Rhyolitic  obsidian  

75.8 

12.4 

0.2 

1.3 

0.3 

0.8 

4.0 

4.6 

0.4 

Rhyolitic  pitchstone  

71.6 

13.1 

0.7 

0.3 

0.1 

0.7 

3.8 

4.1 

5.5 

Andesitic  perlite  

65.1 

15.7 

2.2 

1.9 

1.4 

3.6 

2.9 

4.0 

2.4 

61  2 

18  0 

1   3 

4   5 

0  4 

1  9 

6  5 

5  9 

0  5 

MnO  =  0.4 

53  8 

13  5 

3  0 

7  4 

6  5 

10.3 

3.2 

0.6 

0.6 

TiO  =   0.2 

Basaltic  obsidan  

50.7 

12.0 

3.4 

8.1 

7.3 

12.4 

2.7 

0.2 

0.5 

Ti02  =  1.7 

414        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

those  found  in  various  types  of  igneous  rocks  ranging  from  the 
high  silica  content  of  rhyolite  to  the  relatively  low  silica  content 
of  basalt.  Water  is  low  in  obsidian,  but  comparatively  high  in 
pitchstone. 

Blowpipe  Tests.  The  fusibility  of  volcanic  glass  is  character- 
istic. It  is  fusible  to  a  vesicular  enamel,  but  this  enamel  on 
further  heating  is  infusible,  which  is  due  to  the  fact  that  the 
water  is  driven  off.  In  the  closed  tube  it  gives  more  or  less 
water  (0.5  to  5. per  cent.). 

Insoluble  in  acids.  Tests  for  silica  and  the  metals  can  be 
obtained  by  making  a  sodium  carbonate  fusion  (see  p.  49). 
The  alkalies  must  be  determined  in  a  separate  sample  by  fusion 
with  NH4C1  and  CaCO3. 

Distinguishing  Features.  Opal  is  about  the  only  mineral 
ordinarily  confused  with  volcanic  glass.  These  two  can  easily 
be  distinguished  by  their  fusibility.  (Opal  is  infusible.)  The 
index  of  refraction  of  glass,  though  variable,  is  always  higher 
than  that  of  opal. 

Occurrence.  1.  Volcanic  glass  is  an  igneous  rock  occurring 
in  surface  flows  or  as  selvage  on  lavas  and  occasionally  in  dikes 
(pitchstones).  It  has  been  formed  by  the  rapid  cooling  of 
the  molten  magma,  and  as  a  consequence  some  of  the  water  of 
the  original  magma  is  usually  retained. 

Hydrocarbons 

The  naturally  occurring  hydrocarbons  vary  from  natural  gas 
[largely  methane  (CH4)  with  variable  amounts  of  ethane,  (C2- 
H6),  carbon  dioxid,  nitrogen,  argon,  neon,  and  helium]  through 
liquid  petroleum  (largely  hydrocarbons  of  the  methane  or 
paraffin  series  with  the  general  formula  CnH2n+2)  and  viscous 
maltha  to  solid  hydrocarbons  which  may  be  divided  into  four 
fairly  well-defined  groups,  viz.,  resins,  waxes,  asphaltum,  and 
coal. 

Resins.  The  resins  are  oxygenated  hydrocarbons.  They 
are  amorphous  and  have  a  resinous  luster.  The  specific  gravity 


SILICATES  415 

is  slightly  above  unity  (1.00  to  1.25).  They  burn  or  melt  easily 
and  are  more  or  less  easily  soluble  in  alcohol,  ether,  and 
turpentine. 

Of  the  various  fossil  resins  amber  is  the  best  known  on  account 
of  its  well-known  uses.  It  varies  from  a  pale  yellow  to  deep 
brown  and  has  a  specific  gravity  of  about  1.05.  The  best 
amber  is  found  along  the  Baltic  coast  of  Prussia.  It  has  been 
formed  by  an  extinct  species  of  pine. 

Other  fossil  resins  include  gum  copal  from  Africa  and  kauri 
gum  from  New  Zealand  used  in  varnishes.  There  are  also 
many  local  names  used  for  various  resins. 

Mineral  waxes.  Ozocerite  or  ozokerite  is  the  best  known 
representative  of  this  group  of  hydrocarbons  which  are  natural 
paraffins  with  impurities.  Ozocerite  is  the  name  applied  to  a 
soft  brown  mineral  wax  from  Galicia,  also  found  in  southern 
Utah.  It  is  soluble  in  ether  and  has  a  specific  gravity  of  0.85 
to  0.95.  Refined  ozocerite  is  used  in  the  manufacture  of  candles, 
ointments,  and  as  an  insulator  for  electrical  apparatus. 

Asphaltum  or  asphalt  is  a  general  name  for  a  great  variety  of 
black,  solid,  more  or  less  oxygenated  hydrocarbons.  They 
include  besides  the  well-known  Trinidad  Lake  asphalt,  other 
varieties  which  have  received  special  names  such  as  albertite, 
gilsonite,  grahamite,  manjak,  wurtzilite,  and  many  others  of 
local  importance.  Each  of  these  has  special  characters  of  its 
own,  but  they  are  all  similar  with  a  hardness  of  1  to  2J^,  specific 
gravity  of  1.0  to  1.8.  They  melt  easily  and  burn  with  a  dis- 
agreeable odor.  They  are  more  or  less  soluble  in  alcohol,  ether, 
turpentine,  carbon  bisulfid,  and  chloroform.  The  relative 
solubility  in  these  various  solvents  is  the  best  method  of  dis- 
tinguishing the  various  kinds  of  asphal turns. 

Asphalturn  occurs  in  veins  usually  and  rarely  in  lake  deposits 
as  on  Trinidad  Island.  Asphaltum  and  semi-solid  hydrocarbons 
also  occur  as  impregnation  of  sandstones  or  limestones.  These 
bituminous  sandstones  and  limestones  have  been  used  for 
paving  in  some  parts  of  the  United  States. 


416        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

The  oil  shales  found  so  abundantly  in  western  Colorado 
should  also  be  mentioned  here.  On  distillation  they  yield  gas, 
crude  oil,  and  ammonia.  The  crude  oil  on  refining  furnishes 
gasoline,  burning  oils,  and  paraffin  wax. 

Coal.  Finally  we  have  the  coals  which  vary  from  lignite 
through  subbituminous,  bituminous,  and  semianthracite  to 
anthracite.  The  chemical  compounds  in  coal  are  for  the  most 
part  unknown.  Coal  consists  largely  of  carbon,  hydrogen,  and 
oxygen,  with  small  amounts  of  nitrogen  and  sulfur.  Analyses  are 
usually  given  in  a  proximate  form:  moisture,  volatile  matter, 
fixed  carbon,  and  ash. 

The  specific  gravity  of  coal  varies  from  about  1.2  to  1.7.  The 
hardness  reaches  2  to  2^  in  anthracite. 

Coal  is  distinguished  from  other  hydrocarbons  by  the  fact 
that  it  is  practically  insoluble  in  the  organic  solvents  (ether, 
alcohol,  etc.). 

Coal  occurs  in  beds  and  so  must  be  ranked  as  a  rock  as  well 
as  a  mineraloid.  Anthracite  may  be  said  to  constitute  a  kind 
of  metamorphic  coal. 

Coal  is  known  to  be  of  vegetable  origin.  All  gradations  have 
been  traced  from  peat  through  lignite  into  various  members  of 
the  coal  series.  There  is,  however,  disagreement  as  to  the  details 
of  the  formation  of  coal. 

Salts  of  Organic  Acids.  The  discussion  of  mineraloids  would 
be  incomplete  without  a  brief  reference  to  the  salts  of  certain 
organic  acids.  One  of  these,  calcium  oxalate,  occurs  in  plant 
tissues  and  is  also  found  in  coal  beds  in  monoclinic  crystals  which 
have  received  the  name  whewellite.  They  have  the  composition 
CaC2O4-H2O. 


PART  III 

THE  OCCURRENCE,  ASSOCIATION,  AND  ORIGIN  OF 

MINERALS 

A.  GENERAL  PRINCIPLES 

The  determination  of  the  properties  of  a  mineral  does  not  end 
its  investigation.  There  still  remains  to  be  determined  the 
problem  of  its  role  in  nature.  What  is  its  relation  to  associated 
minerals  and  how  has  it  been  formed?  This  is  to  some  extent  an 
independent  subject,  for  with  the  possible  exception  of  amorphous 
minerals  of  colloidal  origin,  the  essential  properties  of  a  mineral 
are  not  dependent  on  its  previous  source  or  history.  The  facts  of 
occurrence  and  association  are  important  from  the  scientific 
standpoint  and  also  from  the  economic  standpoint  in  case  the 
mineral  or  one  of  its  associates  is  of  commercial  value. 

A  great  many  of  the  subjects  considered  in  the  following  pages 
are  treated  under  the  head  petrography,  the  science  that  deals 
with  rocks  especially  from  the  descriptive  side.  A  broader  treat- 
ment of  the  whole  subject  of  mineral  occurrences,  including 
mineral  deposits  as  well  as  rocks  is  attempted. 

1.  ASSOCIATION  OF  MINERALS 

The  minerals  of  rocks  and  other  mineral  deposits  occur  to- 
gether in  more  or  less  definite  association  one  with  another. 
Many  of  the  associations  are  so  characteristic  that  the  expe- 
rienced mineralogist  makes  use  of  the  facts  in  determining 
minerals.  Franklinite,  for  example,  is  practically  always  asso- 
ciated with  willemite  and  zincite  (ZnO).  Lepidolite,  tourmaline, 
microcline,  albite,  spodumene,  and  beryl  are  characteristic  of 
granite  pegmatites.  Nepheline  occurs  with  the  feldspars  in 
igneous  rocks  and  is  never  found  with  quartz.  The  zeolites 

27  417 


418        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

occur  largely  with  datolite,  apophyllite,  prehnite,  calcite,  chalce- 
dony, and  quartz  as  secondary  minerals  in  basalts  and  related 
rocks.  Chondrodite  is  almost  invariably  found  with  phlogopite 
and  spinel  in  metamorphic  limestones. 

The  term  paragenesis  is  used  as  a  general  term  for  the 
association  of  minerals  with  special  reference  to  their  occurrence 
and  origin. 

2.  ORDER  OF  SUCCESSION 

Not  only  the  association  but  also  the  order  of  succession  of 
minerals  is  often  characteristic.  In  many  copper  ore  deposits,  for 
example,  the  order  in  which  the  minerals  have  been  formed  is  as 
follows:  (1)  pyrite,  (2)  chalcopyrite,  (3)  bornite,  (4)  chalcocite. 
In  most  ore  deposits  the  ore-minerals  have  been  formed  after 
quartz  and  the  silicates,  although  some  of  the  silicates,  among 
them  sericite,  chlorite,  tremolite,  and  antigorite,  are  apparently 
formed  after  the  ore-minerals. 

The  order  of  succession  of  the  minerals  of  a  deposit  combined 
with  the  geology  gives  valuable  clues  to  the  history  of  the 
deposit. 

3.  PROCESSES  OF  MINERAL  FORMATION 

Minerals  may  be  formed  in  various  ways.  Some  have  been 
formed  from  water  solution  (veins,  spring  deposits,  secondary 
minerals  in  cavities)  either  by  concentration  of  solutions  or  by 
chemical  reactions.  Some  have  been  formed  by  separation  from 
a  molten  magma  (minerals  of  the  igneous  rocks) .  The  magma  is 
to  be  looked  upon  as  a  solution  of  certain  compounds  in  others, 
for  the  minerals  separate  out  in  the  order  of  solubility  rather  than 
of  fusibility.  Others  have  been  formed  by  organisms.  Still 
others  have  been  formed  by  the  chemical  readjustment  incident 
to  metamorphism.  A  few  minerals  are  the  result  of  exhalations 
of  gases  in  volcanic  regions. 

4.  SYNTHESIS  OF  MINERALS 

Besides  filling  up  gaps  in  isomorphous  groups  and  furnishing 
better  material  for  study,  the  synthesis  of  a  mineral  often  gives  a 


THE  ORIGIN  OF  MINERALS  419 

clue  to  its  origin  in  nature.  Most  minerals  have  been  produced 
artificially,  but  a  few,  such  as  tourmaline,  have  never  been  pro- 
duced except  in  Nature's  laboratory  itself. 

The  methods  of  mineral  synthesis  differ  greatly;  the  apparatus 
varies  from  a  test-tube  to  the  electric  furnace.  A  general 
method  is  that  of  the  sealed  tube.  A  hard  glass  tube  containing 
the  proper  substances  is  sealed  up  and  heated  in  a  bomb-furnace 
for  several  hours  or  days,  or  even  weeks  if  necessary.  Water 
vapor  under  pressure  plays  an  important  part  in  reaction,  as  it 
often  seems  to  in  nature.  An  example  of  this  method  is  the  pro- 
duction of  artificial  covellite  (CuS)  by  heating  powdered  sphaler- 
ite (ZnS)  in  a  water  solution  of  copper  sulfate.  An  atmosphere 
of  CO2  is  used  to  prevent  oxidation.  After  a  few  hours  a  blue- 
black  powder  (CuS)  appears.  The  reaction  is  ZnS  +  CuSC>4  = 
CuS  +  ZnSO4.  This  experiment  was  performed  by  the  author 
in  an  attempt  to  explain  covellite  pseudomorphs  after  sphalerite 
found  by  him  in  the  Joplin  district.  In  such  experiments  geologic 
time  is  compensated  for  by  increased  temperature  and  pressure. 

Some  of  the  " basic"  igneous  rocks  are  easily  reproduced,  and 
such  minerals  as  olivine,  pyroxene,  leucite,  and  plagioclase  crystal- 
lize out  from  a  molten  mass  of  the  proper  constituents.  At- 
tempts to  reproduce  the  "acid"  igneous  rocks  on  the  other  hand 
are  not  successful,  for  the  magma  solidifies  as  a  glass.  The  lack  of 
gases  which  were  present  in  the  natural  magma  accounts  for  the 
failure. 

French  mineralogists  and  chemists  have  been  especially  active 
in  mineral  synthesis.  Moissan  produced  diamond  by  dissolving 
carbon  in  molten  iron  and  the  plunging  the  mass  into  water. 
Artificial  rubies,  sapphires,  and  other  colored  varieties  of  corun- 
dum are  now  made  in  Paris  on  a  commercial  scale.  These  were 
first  successfully  produced  by  Verneuil  in  1904.  Except  for 
very  minute  bubbles  they  have  exactly  the  same  physical  prop- 
erties as  the  natural  gems  and  are  distinguished  from  them 
with  great  difficulty.  Artificial  stones  resembling  the  emerald  are 
easy  to  produce  but  they  are  really  glass  and  not  true  artificial 
beryl. 


420        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Intermediate  between  the  naturally  occurring  minerals  and  the 
so-called  artificial  minerals  are  the  mineral  substances  formed 
on  mine-tools,  prehistoric  implements,  old  coins,  etc.  Man  has 
unintentionally  furnished  part  of  the  material,  but  has  not  di- 
rected the  conditions  of  the  experiments,  hence  the  term  accidental 
synthesis.  The  author  has  identified  cuprite,  copper,  malachite, 
azurite,  and  cerussite  on  buried  Chinese  coins  of  the  seventh 
century  found  at  Kiukiang,  China.  In  the  old  Roman  baths 
at  the  hot  springs  of  Bourbonne-les-Bains,  France,  bronze  coins 
thrown  in  the  spring  as  votive  offerings  were  found  by  Daubree 
to  be  incrusted  with  such  minerals  as  chalcocite,  chalcopyrite, 
bornite,  and  tetrahedrite,  and  the  conduits  leading  to  the  baths 
were  lined  with  zeolites. 

6.  ALTERATION  AND  REPLACEMENT  OF  MINERALS 

A  mineral  formed  under  one  set  of  conditions  may  be  unstable 
under  another  set  of  conditions.  .  This  accounts  for  the  observed 
replacement  of  one  mineral  by  another.  Pyrite,  for  example,  is 
unstable  under  oxidizing  conditions  and  so  in  the  oxidized  zone  we 
usually  find  it  more  or  less  altered  to  limonite,  turyite,  or  some- 
times to  iron  sulfates.  If  copper  solutions  are  present  it  may  be 
altered  to  chalcocite.  Replacements  in  which  there  is  a  chemical 
relation  between  the  original  mineral  and  the  replacing  mineral 
are  called  alterations.  The  more  common  chemical  changes 
involved  in  alteration  are  oxidation  (sulfids  to  sulfates,  sulfids  to 
oxids,  arsenids  to  arsenates,  etc.),  reduction  (sulfates  to  sulfids, 
sulfids  to  metals,  oxids  to  metals),  carbonation  (sulfids  to  car- 
bonates), and  hydra tion  (anhydrous  salts  to  hydrous  salts). 
There  are  also  more  complex  changes,  some  of  which  have  received 
special  names.  The  following  may  be  mentioned  as  prominent 
examples  of  alterations:  pyrite  to  limonite;  galena  to  cerussite, 
usually  through  the  intermediate  stage  of  anglesite ;  sphalerite  to 
smithsonite;  bornite  to  chalcocite;  copper  to  cuprite;  calcite  to 
smithsonite;  olivine  to  antigorite  (serpentinization) ;  pyroxene 
to  actinolite  (uralitization) ;  and  feldspars  to  sericite  (sericitiza- 
tion). 


THE  ORIGIN  OF  MINERALS  421 

One  of  the  best  evidences  of  alteration  and  replacement  is  the 
occurrence  of  pseudomorphs.  A  pseudomorph  is  one  mineral 
with  the  form  of  another,  a  false  form  as  the  name  indicates. 
Thus  limonite,  an  amorphous  mineral,  is  often  found  in  cubes. 
The  explanation  is  that  the  cubes  were  originally  pyrite  and  were 
altered  to  limonite  by  oxidation  and  hydration.  Such  a  specimen 
is  said  to  be  a  limonite  pseudomorph  after  pyrite.  Four  general 
classes  of  pseudomorphs  are  recognized : 

1.  Alteration  pseudomorphs  with  either  a  loss,  gain,  or  inter- 
change of  some  constituent.     Examples  of  the  three  cases  re- 
spectively: copper  after  azurite;  malachite  after  cuprite  ;cerussite 
after  galena. 

2.  Paramorphic  pseudomorphs  or  par  amor  phs.     A  pseudomorph 
of  one  polymorphous  mineral  after  another  is  called  a  paramorph. 
Example:  calcite  after  aragonite. 

3.  Substitution    pseudomorphs.     An   interchange  of  substance 
not  involving  alteration.     Example:  chalcedony  after  calcite. 

4.  Incrustation  pseudomorphs.     If  one  mineral  incrusts  another 
and  then  the  original  mineral  is  dissolved  there  remains  a  cavity 
which  may  afterward  be  filled  by  still  another  mineral.     Ex- 
ample: quartz  after  fluorite. 

The  replacement  of  a  fossil  by  a  mineral  is  called  a  petrifaction. 

The  more  common  minerals  occurring  as  petrifactions  are 
calcite,  chalcedony,  opal,  quartz,  limonite,  pyrite,  and  cellophane, 
and  rarely  such  minerals  as  barite  and  sphalerite. 

Fossil  wood  is  usually  preserved  as  opal  and  chalcedony, 
occasionally  as  quartz  and  rarely  as  calcite  or  dolomite.  In 
New  Mexico  cuprified  wood  now  made  up  of  hematite,  pyrite, 
bornite,  chalcocite,  and  melaconite  (amorphous  CuO)  is  com- 
mon in  certain  regions.  The  cell-structure  of  the  wood  is  often 
preserved  and  sometimes  the  wood  may  be  identified. 

Fossil  bone,  as  the  author  has  recently  discovered,  is  made  up 
almost  entirely  of  the  mineral  cellophane.  The  structure  of  the 
bone  is  usually  perfectly  preserved.  The  original  bone  consists 
of  calcium  carbonophosphate  and  an  organic  substance  called 


422        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

ossein,  one  of  the  proteins.  The  fossilization  of  the  bone  con- 
sists of  the  elimination  of  the  organic  substance  (some  of  it 
usually  remains  behind  as  a  pigment)  and  an  enrichment  of  the 
inorganic  portion. 

Replacement  may  take  place  on  an  extensive  scale  and  in  this 
way  some  of  our  prominent  types  of  ore-deposits  have  been 
formed.  Whole  rock  formations  may  be  replaced  by  solutions. 
Many  phosphorites,  for  example,  have  been  formed  by  the  action 
of  phosphatic  solutions  upon  limestones.  Limestones  may  also 
be  replaced  by  silica,  usually  in  the  form  of  chalcedony. 

B.  MINERAL  OCCURRENCES  INCLUDING  ROCKS 

No  satisfactory  classification  of  the  various  mineral  occurrences 
can  be  made  on  account  of  the  multiplicity  of  factors  to  be  taken 
into  account.  For  convenience,  rocks  and  mineral  deposits  are 
treated  under  the  following  heads. 

1.  IGNEOUS  ROCKS 

General  Discussion 

The  igneous  rocks  include  all  rocks  that  are  the  result  of 
solidification  of  molten  material  on  or  in  the  earth's  crust  or 
outer  shell.  They  are  of  especial  importance  because  of  the 
fact  that  they  are  the  original  source  of  the  other  two  great 
groups  of  rocks,  the  sedimentary  and  the  metamorphic.  Igneous 
rocks  are  characterized  by  the  presence  of  certain  minerals 
which  are  practically  absent  from  other  rocks,  by  the  massive 
appearance  or  absence  of  stratification  and  foliation,  and  also 
by  the  absence  of  fossils. 

The  prominent  features  of  igneous  rocks  may  be  discussed 
under  the  following  heads:  (1)  chemical  composition,  (2)  mineral 
composition,  (3)  texture,  (4)  structure,  and  (5)  mode  of  occur- 
rence. These  are  the  factors  used  in  the  description  and  classifi- 
cation of  igneous  rocks. 


THE  ORIGIN  OF  MINERALS 


423 


Chemical  Composition. 

Chemical  analyses  of  igneous  rocks  are  always  recorded  in  the 
form  of  oxids.  The  nine  principal  oxids  are:  silica  (SiO2), 
alumina  (A12O3),  ferric  oxid  (Fe203),  ferrous  oxid  (FeO),  mag- 
nesia (MgO),  lime  (CaO),  soda  (Na2O),  potassa  (K2O),  and 
water  (H2O).  Practically  all -igneous  rocks  also  contain  small 
amounts  of  titania  (TiO2),  carbon  dioxid  (CO2),  baryta  (BaO), 
manganous  oxid  (MnO),  and  phosphoric  anhydrid  (P2O5).  The 
oxids  are  in  chemical  combination  and  do  not  exist  free  except 
in  a  few  cases,  the  most  prominent  of  which  is  free  silica 
in  the  form  of  quartz.  Among  the  various  constituents  silica 
predominates.  It  varies  from  a  maximum  of  about  75  per  cent, 
in  granites  and  rhyolites  to  a  minimum  of  about  40  per  cent,  in 
peridotites.  Igneous  rocks  high  in  silica  are  persilicic  (the  so-called 
acid  rocks),  those  low  in  silica,  subsilicic  (the  so-called  basic 
rocks),  and  those  of  intermediate  silica  content,  mediosilicic. 
Granites  and  rhyolites,  for  example,  are  persilicic  rocks,  basalts, 
olivine  gabbros,  and  peridotites,  subsilicic  rocks.  Alumina  in 
general  is  fairly  constant,  but  is  very  low  in  peridotites 
and  high  in  syenites.  The  iron  oxids,  magnesia,  and  lime  are 
low  in  persilicic  rocks  and  high  in  the  subsilicic  rocks.  The 
alkalies  (soda  and  potassa),  on  the  other  hand,  are  relatively 
high  in  persilicic  rocks  and  almost  lacking  in  the  peridotites. 

The  following  list  of  analyses  will  serve  to  show  the  range  in 
chemical  composition  of  the  more  common  types  of  igneous  rocks. 


i 

s 

<j 

I 

°£ 

o 

H 

? 

0 

i 

fc 

° 

M 

O 

w 

d 

§ 

Granite  (Pike's  Peak)  

73.9 

13.6 

0.3 

0.4 

0.1 

0.2 

2.5 

8.0 

0.5 

0.1 

Granodiorite  (Mariposa  Co.,  Cal.)  

66.3 

16.0 

1.8 

1.9 

1.1 

3.7 

4.1 

3.5 

0.5 

0,9 

Syenite  (Plauen,  Saxony)  

62.5 

16.5 

2.4 

2.0 

1.9 

4.2 

4.4 

4.6 

0.6 

1.3 

Diorite  (Electric  Peak)  .  .  ,  

58  1 

18  0 

2.5 

4   6 

3   5 

6  2 

3   6 

2  2 

0  9 

1   2 

Gabbro  (Island  of  Skye)    .  . 

52  8 

17  8 

1   2 

4   8 

4  8 

12  9 

3  0 

0  5 

1  2 

0  5 

Olivine  gabbro  (Birch)  

45.7 

16.4 

0.7 

13.9 

11.6 

7.3 

2.1 

0.4 

0.9 

1.1 

Peridotite  (Devonshire)  

40.1 

7.8 

7.3 

8.6 

23.7 

6.5 

1.2 

0.5 

4.0 

0.6 

Igneous  rocks  are  the  result  of  the  cooling  and  consequent 


424        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

solidification  of  molten  material  called  a  magma.  The  original 
magma  may  be  observed  in  active  volcanoes.  If  the  magma  is 
cooled  rapidly  a  glass  is  the  result,  but  if  the  cooling  is  slow  then 
crystallization  takes  place  and  crystals  of  various  minerals 
are  produced. 

During  the  act  of  crystallization,  which  may  be  either  very 
slow  or  relatively  rapid,  certain  oxids  combine  to  form  silicates. 
The  alkalies  and  lime  combine  with  silica  and  alumina  to  form  the 
feldspars;  lime,  magnesia,  and  the  iron  oxids  combine  with  silica 
and  alumina  to  form  the  ferromagnesian  minerals,  a  collective 
term  for  pyroxene,  hornblende,  biotite,  and  olivine.  Some  of  the 
ferric  oxid  combines  with  ferrous  oxid  to  form  magnetite.  Silica 
remaining  after  these  combinations  have  taken  place  crystallizes 
out  in  the  form  of  quartz.  In  rare  cases  there  is  too  much  alu- 
mina to  combine  with  silica  and  the  alkalies  and  the  other 
constituents  and  so  corundum  is  formed.  It  must  be  admitted 
that  this  process  is  very  complicated,  especially  when  gases  are 
present,  for  it  is  not  always  possible  to  predict  the  mineral  com- 
position from  a  chemical  analysis. 

Mineral  Composition.  Comparatively  few  minerals  constitute 
the  bulk  of  igneous  rocks.  Only  a  dozen  or  two  minerals  are 
found  in  quantity  in  the  various  igneous  rocks  even  if  we  in- 
clude the  less  common  and  unusual  rock  types. 

The  more  important  minerals  of  igneous  rocks  are: 

1.  Essential    Minerals.     Feldspars    (orthoclase    and    plagio- 
clase),  pyroxene  (augite,  diopside,  and  hypersthene) ,  hornblende, 
biotite,  olivine,  quartz,  nepheline  (rare),  and  leucite  (rare). 

2.  Accessory  Minerals.     Magnetite,  apatite,  titanite,  ilmenite, 
and  zircon. 

3.  Secondary    Minerals.     Quartz,    chalcedony,    opal,    calcite, 
zeolites,  chlorite,  sericite,  antigorite,  and  kaolinite. 

Texture.  In  addition  to  the  chemical  and  mineralogical 
composition,  the  size,  shape,  and  arrangement  of  the  constituent 
minerals  must  be  taken  into  account  in  the  study  of  rocks.  These 
features  of  a  rock  are  collectively  known  as  the  texture.  The 


THE  ORIGIN  OF  MINERALS  425 

texture  is  important  because  it  gives  some  indication  of  the  phys- 
ical conditions  obtaining  during  the  formation  of  the  rock.  A 
magma  with  a  given  chemical  composition  may  form  several 
distinct  rock  types  depending  upon  the  temperature,  hydro- 
static pressure,  rate  of  cooling,  presence  of  gases,  etc. 

Three  main  factors  are  involved  in  texture.  In  the  first  place 
there  is  the  degree  of  crystallization.  The  rock  may  be  made  up 
entirely  of  glass,  entirely  of  crystals,  or  any  given  proportion 
of  the  two.  A  second  factor  is  the  magnitude  of  the  crystals. 
The  crystals  present  may  be  very  minute,  small,  or  large,  thus 
furnishing  fine-grained  rocks  (less  than  1  mm.  in  size),  medium- 
grained  rocks  (between  1  mm.  and  5  mm.  in  size),  and  coarse- 
grained rocks  (greater  than  5  mm.  in  size).  The  third  factor  is 
fabric,  a  term  which  denotes  the  relative  size  of  the  crystals  and 
their  shape  and  arrangement.  If  the  crystals  are  of  the  same 
order  of  magnitude  the  rock  is  said  to  be  even-grained.  If  the 
crystals  are  of  different  order  of  magnitude  the  fabric  is  said  to  be 
porphyritic.  In  this  case  the  larger  crystals  are  called  pheno- 
crysts  and  their  matrix,  which  may  contain  more  or  less  glass,  is 
called  the  groundmass.  The  crystals  themselves  may  be  euhe- 
dral,  subhedral,  or  anhedral,  and  if  euhedral  may  be  tabular, 
prismatic,  or  equant  (equidimensional)  in  habit.  The  crystals 
may  have  a  more  or  less  parallel  arrangement  or  may  inter- 
penetrate each  other  and  thus  form  the  graphic  texture.  In  the 
volcanic  glasses  spherulitic  aggregates  of  the  feldspars  are  often 
present. 

Structure.  The  larger  features  of  igneous  rocks  are  included 
under  the  term  structure.  It  is  obvious  that  no  clear  distinction 
can  be  made  between  the  terms  texture  and  structure.  Most  of 
the  igneous  rocks  have  a  massive  structure  but  in  some  of  them 
banded,  columnar,  or  spheroidal  structures  are  present.  These 
result  largely  from  contraction  on  cooling.  Many  of  the  volcanic 
rocks,  especially  basalts,  have  a  vesicular  structure.  The  rock  is 
filled  with  rounded  cavities  which  are  due  to  the  escape  of  steam 
and  other  gases  from  the  rock  during  its  consolidation.  If  these 


426        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

cavities  at  a  later  time  are  filled  with  minerals  such  as  the 
zeolites,  calcite,  chalcedony,  quartz,  etc.,  the  structure  is  said 
to  be  amygdaloidal  (from  the  Greek  word  for  almond) . 

Mode  of  Occurrence.  According  to  the  geological  mode  of 
occurrence  igneous  rock  masses  may  be  divided  into  three  large 
groups;  (1)  extrusive,  (2)  injected,  and  (3)  subjacent. 

Extrusive  rocks  are  igneous  rocks  that  have  been  formed  by  the 
cooling  of  magmas  on  or  very  near  the  earth's  surface.  They 
are  the  well-known  lavas  found  around  recent  volcanoes.  The 
terms  lava  and  volcanic  rock  should  be  restricted  to  extrusive 
rocks  and  should  not  be  used  as  a  synonym  of  igneous  rock  in 
general.  The  extrusive  rocks  comprise  two  general  classes: 
fissure  eruptions  and  central  eruptions.  The  great  basaltic 
lava  fields  of  eastern  Oregon  and  Idaho,  northwestern  India,  and 
Iceland  are  fissure  eruptions.  The  lavas  have  issued  quietly 
through  narrow  fissures  and  probably  were  very  fluid  at  the 
time  of  extrusion.  Under  central  eruptions  are  included  rock 
bodies  formed  by  extrusion  from  a  single  vent.  They  comprise 
volcanic  cones,  plugs,  and  surface  flows.  The  lavas  formed 
during  quiescent  periods  often  alternate  with  beds  of  fragmental 
material,  volcanic  ash,  which  are  due  to  explosive  action.  When 
consolidated  the  beds  of  volcanic  ash  become  tuffs.  The  volcanic 
neck  feeding  the  vent  may  be  exposed  by  erosion. 

The  injected  rocks  form  the  smaller  intrusions  such  as  dikes, 
sills,  and  laccoliths.  A  dike  is  formed  by  the  injection  of  a  magma 
into  a  relatively  narrow  fissure  not  parallel  to  the  bedding  plane 
of  a  sedimentary  rock.  If  the  fissure  is  parallel  to  the  bedding 
plane  a  sill  is  produced.  The  term  laccolith  is  used  for  a  dome- 
shaped  intrusion  which  has  bowed  up  the  overlying  strata. 

The  subjacent  rock  bodies  comprise  batholiths  and  stocks.  A 
batholith  is  a  large  rock  mass  of  indeterminate  shape  and  size 
exposed  by  erosion  in  mountainous  regions.  Similar,  but  smaller, 
masses  are  called  stocks.  They  probably  represent  protruding 
portions  of  irregular  batholiths.  The  formation  of  batholiths 
is  a  disputed  question  in  geology.  They  may  be  injected  but  it 


THE  ORIGIN  OF  MINERALS  427 

is  better  to  use  the  term  subjacent,  proposed  by  Daly,  which 
is  non-committal  as  to  origin. 

The  Classification  of  Igneous  Rocks 

A  good  many  different  classifications  of  igneous  rocks  have 
been  proposed,  but  none  can  be  considered  satisfactory.  There  is 
much  difference  as  to  the  relative  value  of  the  criteria  used  in 
the  classification.  Some  emphasize  texture  while  others  almost 
disregard  it,  and  in  one  widely  used  classification  the  geological 
mode  of  occurrence  is  the  prominent  feature.  The  principal 
difficulty  in  the  classification  of  igneous  rocks  is  the  fact  that  they 
intergrade  in  all  directions.  They  are  aggregates  of  minerals  in 
all  proportions  so  that  division  lines  between  groups  are  neces- 
sarily arbitrary.  It  is  necessary  to  establish  certain  arbitrary 
rock  types  and  classify  these  in  the  most  convenient  way. 

For  megascopic  and  microscopic  work  in  the  absence  of  chemical 
analyses,  mineral  composition  and  texture  are  the  two  principal 
criteria  used  in  the  classification.  The  following  diagram  gives  a 
convenient  simple  classification  of  the  more  common  igneous 
rock  types.  Two  main  textures  are  recognized.  In  the  grained 
rocks  practically  all  the  constituent  minerals  are  large  enough  to 
be  seen  with  the  unaided  eye,  while  in  the  dense  rocks  at  least 
some  of  the  constituents  are  too  small  to  be  seen  without  the 
microscope  (more  or  less  glass  may  be  present).  Porphyritic 
and  non-porphyritic  varieties  of  each  class  are  given  so  that 
four  kinds  of  texture  are  recognized.  These  extend  in  a  hori- 
zontal direction. 

The  dense  rocks  (trachyte,  rhyolite,  andesite,  auganite,  and 
basalt)  generally  occur  in  surface  flows,  volcanic  necks,  etc.  The 
grained  rocks  (syenite,  granite,  diorite,  etc.)  generally  occur  in 
stocks  and  batholiths  and  in  the  centers  of  large  dikes  and  lacco- 
liths. The  intermediate  porphyries  occur  in  the  smaller  intru- 
sions and  in  the  interior  of  thick  surface  flows.  Thus  there  is  a 
general  correlation  between  the  texture  and  mode  of  occurrence 
but  too  much  stress  should  not  be  laid  upon  it,  for  the  correlation 


428        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


THE  ORIGIN  OF  MINERALS  429 

is  only  a  general  one.  The  classification  of  the  rock  should  be 
based  upon  its  intrinsic  characters,  that  is  upon  characters  that 
can  be  determined  from  the  specimen  itself.  The  geological 
mode  of  occurrence  is  a  question  of  field  geology.  These  two 
sets  of  equally  important  facts  should  be  determined  independ- 
ently. If  this  is  not  done,  incorrect  conclusions  may  be  drawn. 

The  vertical  columns,  on  the  other  hand,  contain  all  the  rocks 
with  a  given  mineral  composition  and  generally  speaking  those 
with  about  the  same  chemical  composition. 

A  Granite -Rhyolite  Series. 

This  series  includes  all  igneous  rocks  composed  essentially  of 
quartz  and  orthoclase  with  minor  amounts  of  ferromagnesian 
minerals,  and  the  corresponding  glassy  equivalents. 

Granite  is  perhaps  the  most  familiar  of  all  igneous  rocks  because 
of  its  wide  use  as  a  building  and  ornamental  stone.  The  two 
essential  minerals  of  granite  are  quartz  and  orthoclase  feldspar. 
Mica  may  or  may  not  be  present.  Plagioclase  is  often  present  in 
addition  to  orthoclase  and  is  distinguished  from  orthoclase  by 
twinning  striations  and  often  by  a  color  difference.  The  feld- 
spars are  white,  gray,  pink,  or  red,  and  largely  determine  the  color 
of  the  rock,  for  the  quartz  is  usually  transparent.  The  ferromag- 
nesian minerals  commonly  present  are  hornblende  and  biotite. 
Muscovite  also  is  sometimes  present.  Rarer  constituents  include 
epidote  and  tourmaline.  The  minor  accessories  are  apatite,  zir- 
con, titanite,  and  magnetite. 

The  chemical  composition  of  an  average  granite  is  shown  in  the 
tabulation  on  page  423.  Compared  with  other  igneous  rocks 
it  is  high  in  silica  and  alkalies. 

The  characteristic  occurrence  of  granite  is  in  batholiths  and 
stocks. 

With  decrease  'in  the  amount  of  ferromagnesian  minerals 
normal  granite  grades  into  a  variety  called  alaskite.  With 
increase  in  the  amount  of  ferromagnesian  minerals  and  consequent 
decrease  in  the  quartz  and  orthoclase  it  passes  into  granodiorite. 


430        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

With  decrease  in  the  amount  of  quartz  and  increase  in  orthoclase 
it  passes  through  quartz  syenite  into  syenite. 

With  decrease  in  the  size  of  the  mineral  grains  and  the  appear- 
ance of  one  or  more  of  the  minerals  in  crystals  of  two  generations 
the  granites  grade  imperceptibly  through  granite  porphyries 
and  rhyolite  porphyries  into  rhy elites. 

Rhyolite.  The  rhyolites  are  the  dense  or  fine-grained  equiva- 
lents of  granites.  The  texture  varies  from  porphyritic  through 
felsitic  into  glassy.  Flow  structure  is  prominent  in  many  rhyo- 
lites and  they  are  often  more  or  less  cellular.  Spherulites  are  a 
prominent  feature  of  some  rhyolites.  They  vary  from  microscopic 
size  to  very  large  dimensions.  Rhyolites  are  usually  light  colored 
but  may  be  dark  red  or  even  black  at  times. 

Except  for  the  fact  that  they  contain  more  or  less  glass,  the 
rhyolites  have  about  the  same  constituents  as  the  granites.  The 
quartz,  orthoclase,  and  biotite  occur  in  part  in  well-formed 
crystals  called  phenocrysts.  The  quartz  crystals  have  a  bipy- 
ramidal  habit  (see  Fig.  419,  p.  259)  different  from  that  of  vein 
quartz  (Figs.  416-8).  In  fact  it  is,  or  rather  was  at  the  time  of 
its  formation,  a  different  kind  of  silica  known  as  0-quartz,  but  on 
cooling  it  changes  to  ordinary  or  a-quartz.  The  orthoclase  if 
unaltered  occurs  in  the  transparent  variety  known  as  sanidine. 
The  dense  groundmass,  which  greatly  resembles  chalcedony, 
proves  on  microscopic  examination  to  be  a  mosaic  of  orthoclase 
and  quartz  in  a  matrix  of  more  or  less  glass. 

The  rhyolites  proper  grade  into  volcanic  glasses,  the  principal 
varieties  of  which  are  obsidian  (vitreous  luster),  pitchstone 
(resinous  luster),  perlite  (curved  fracture),  and  pumice  (highly 
vesicular).  Some  of  the  volcanic  glasses  contain  phenocrysts 
and  if  these  are  prominent  the  rock  is  called  a  vitrophyre.  As 
these  glasses  may  correspond  to  trachytes,  andesites,  etc.,  the 
term  rhyolitic,  strictly  speaking,  should  be  used  as  a  prefix 
but  as  a  matter  of  fact  the  great  majority  of  volcanic  glasses 
have  the  composition  of  rhyolite.  [For  a  description  of  glass  see 
page  413.] 


THE  ORIGIN  OF  MINERALS  431 

(6)  Syenite— Trachyte  Series. 

The  rocks  of  this  series  include  all  those  with  dominant  alkali 
feldspar  (orthoclase,  microcline,  or  albite)  which  at  the  same  time 
lack  quartz.  The  glassy  equivalents  are  also  included.  The 
rocks  of  this  group  are  rare,  taken  the  world  over,  but  they  are 
very  interesting  as  they  often  contain  rare  minerals. 

Syenite  is  a  grained  rock  consisting  largely  of  orthoclase  or 
microcline  with  minor  amounts  of  hornblende,  biotite,  and 
pyroxene  and  also  such  accessories  as  titanite,  apatite  and  zircon. 
Nepheline  is  often  present  and  if  in  sufficient  quantity  the 
rock  is  called  nepheline  syenite.  The  nepheline  may  be  recog- 
nized by  its  greasy  luster,  absence  of  cleavage,  inferior  hardness 
(it  is  scratched  by  a  knife  blade),  and  solubility  in  HC1  The 
nepheline  syenites  frequently  contain  sodalite.  Some  varieties 
of  syenite  contain  corundum  as  an  original  mineral.  The  origin 
of  the  corundum  syenites  may  be  explained  by  the  fact  that  the 
original  magma  contained  an  excess  of  A^Os  over  that  required 
for  silica,  alkalies,  and  other  oxids.  After  these  affinities  were 
satisfied,  the  excess  of  A1203  crystallized  out  as  corundum. 

In  Arkansas  syenite  has  been  used  as  a  building  stone.  It 
resists  fire  better  than  granite  because  of  the  more  uniform 
expansion  due  to  the  presence  of  practically  one  mineral  instead 
of  two  minerals  with  unequal  coefficients  of  expansion. 

With  increase  in  the  amount  of  plagioclase  and  ferromagne- 
sian  minerals  and  decrease  in  the  amount  of  orthoclase  the 
syenites  pass  through  intermediate  rocks  known  as  monzonites, 
which  contain  approximately  equal  amounts  of  alkali  feldspar 
and  soda-lime  feldspar,  into  diorites. 

In  texture  the  syenites  grade  through  syenite  'porphyries  and 
trachyte  porphyries  into  trachytes.  (Note  that  this  name  ends 
in  -yte  instead  of  -ite). 

Trachytes  are  the  porphyritic  to  felsitic  equivalents  of  the 
syenites,  for  they  contain  practically  the  same  minerals.  Quartz 
is  lacking  though  its  polymorph  tridymite  is  occasionally  present. 
The  feldspar  phenocrysts  are  largely  sanidine,  the  transparent 
variety  of  orthoclase,  though  plagioclase  is  sometimes  also  present, 


432        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

The  fine-grained  equivalent  of  nepheline  syenite  is  called 
phonolite,  a  rare  type  of  igneous  rock.  Leucite-bearing  trachytes 
are  also  known.  They  are  prominent  in  certain  Italian  volcanic 
regions.  The  fine-grained  equivalent  of  monzonite  is  called 
latite. 

The  glassy  equivalents  of  trachytes  are 'rare  in  comparison 
with  the  rhyolitic  glasses. 

(c)  Granodiorite-Dacite  Series. 

The  rocks  of  this  series  are  intermediate  chemically  and 
mineralogically  between  the  granite-rhyolite  series  and  the  diorite- 
andesite  series. 

Granodiorite  is  a  grained  rock  containing  both  alkali  feldspar 
and  lime-soda  feldspar  together  with  some  quartz  and  fair 
amounts  of  ferromagnesian  minerals.  As  the  name  implies,  it  is 
intermediate  between  granite  and  diorite.  There  is  too  much 
quartz  for  a  diorite  but  not  enough  for  a  granite.  It  is  close  to 
a  quartz  monzonite,  another  intermediate  rock  type  from  which 
it  can  be  distinguished  only  by  a  quanitative  analysis.  The 
granodiorites  grade  through  quartz  diorites  into  diorites. 

Granodiorites  are  prominent  rocks  in  the  great  batholiths 
of  the  Western  United  States,  for  example  in  the  Sierra  Nevada 
of  California. 

Dacite.  The  dense  equivalent  of  granodiorite  may  for  con- 
venience be  called  dacite,  though  the  term  is  usually  employed 
for  a  rock  of  a  little  lower  silica  content,  the  equivalent  of  quartz 
diorite.  Dacite  can  usually  be  recognized  by  the  presence  of 
phenocrysts  of  quartz  together  with  those  of  both  orthoclase 
and  plagioclase.  Dacites  are  wide-spread  rocks  but  do  not 
form  such  large  masses  as  the  andesites  or  rhyolites. 

Granodiorite  porphyry  and  dacite  porphyry  are  intermediate 
in  texture  between  granodiorite  and  dacite. 

(d)  Diorite -Ande site  Series. 

The  diorite-andesite  series  includes  all  igneous  rocks  in  which  the 
essential  minerals  are  sodic  plagioclase  and  some  ferromagnesian 


THE  ORIGIN  OF  MINERALS  433 

minerals  and  their  glassy  equivalents.  They  are  medio-silicic 
rocks  with  silica  content  of  about  55  to  60  per  cent. 

Diorite  is  a  grained  rock  with  about  equal  quantities  of  light  and 
dark  colored  minerals.  The  light  colored  mineral  is  plagioclase 
(oligoclase  or  andesine)  and  the  dark  colored  minerals  usually  horn- 
blende, augite,  or  biotite.  Under  the  diorites  are  included  only 
those  rocks  in  which  the  plagioclase  is  more  sodic  than  Abi  Ani. 
This  makes  the  distinction  between  diorite  and  gabbro  rest 
upon  the  character  of  the  plagioclase.  In  gabbro  the  plagioclase 
is  more  calcic  than  AbiAni.  This  means  that  the  character 
of  the  plagioclase  in  many  cases  must  be  determined  by  optical 
tests  but  after  some  experience  the  two  rock  types  may  often 
be  distinguished  megascopically. 

Some  varieties  of  diorite  contain  a  little  quartz  (quartz  diorite) 
and  thus  grade  into  granodiorite. 

Between  the  diorites  and  andesites  we  have  diorite  porphyries 
and  andesite  porphyries,  names  based  purely  on  differences  in 
texture. 

Andesites  are  dense  equivalents  of  the  diorites  and  so  the 
feldspar  is  oligoclase  or  andesine  (note  that  the  name  of  this 
feldspar  ends  in  -ine  and  the  rock  name  in  -ite).  The  ferro- 
magnesian  mineral  is  either  biotite,  hornblende,  hypersthene,  or 
augite.  Quartz  is  typically  absent  but  the  rocks  between 
andesite  and  dacite  may  contain  a  little  quartz.  With  increase 
of  orthoclase  and  decrease  of  ferromagnesian  minerals  they 
pass  through  latites  into  trachytes  and  with  increase  of  lime  con- 
tent of  the  plagioclase  they  pass  into  auganites. 

The  andesites  are  prominent  and  wide  spread  rocks  in  both 
North  America  and  South  America.  Mt.  Shasta  in  northern 
California  consists  largely  of  andesite. 

(e)  Gabbro-Auganite  Series. 

The  gabbro-auganite  series  includes  igneous  rocks  with  calcic 
plagioclase  and  some  ferromagnesian  mineral  besides  olivine  as 
the  essential  minerals.  The  absence  of  olivine  distinguishes  this 
series  from  the  olivine  gabbro-basalt  series. 

28 


434        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Gabbro  is  here  used  for  an  olivine-free  grained  rock  consisting 
essentially  -of  calcic  plagioclase  (labradorite  or  bytownite)  and 
augite,  hypersthene,  or  hornblende.1  The  gabbros  are  usually 
coarser  grained  than  diorites  and  the  plagioclase  is  often  altered 
to  the  mixture  of  minerals  known  as  saussurite.  The  pyroxene 
may  be  altered  to  an  amphibole  called  uralite. 

Varieties  rich  in  hypersthene  are  called  norite.  With  increase 
in  the  plagioclase  content  the  gabbros  grade  into  anorthosites 
or  labradorite  rocks,  which  are  sometimes  used  as.  an  ornamental 
stone.  With  decrease  in  the  plagioclase  on  the  other  hand,  they 
pass  into  the  pyroxenites,  igneous  rock  composed  essentially  of 
pyroxene.  Quartz  and  orthoclase  are  present  in  some  gabbros 
but  such  types  are  rare. 

With  decrease  in  the  size  of  the  mineral  grains  the  gabbros 
grade  through  gabbro  porphyries  into  auganite  porphyries  and 
auganites. 

Auganite.  This  recently  introduced  term  (A.  N.  Winchell, 
1912)  is  a  useful  one  for  the  dense  equivalent  of  the  olivine-free 
gabbro.  Auganite  differs  from  andesite  in  the  fact  that  the  plagio- 
clase is  more  calcic  than  Abi  Ani,  and  from  basalt  in  the  fact  that 
olivine  is  lacking.  It  has  usually  been  called  augite  andesite  or 
olivine-free  basalt,  but  the  first  name  may  be  used  for  augite- 
bearing  rocks  with  sodic  plagioclase,  and  the  name  basalt  may 
then  be  restricted  to  olivine-bearing  rocks. 

Leu  cite  and  nepheline  occur  in  certain  rare  rocks  of  this  group. 
One  type,  leucite  tephrite,  is  abundant  in  certain  parts  of  Italy. 

(/)  Olivine  Gabbro -Basalt  Series. 

The  olivine  gabbro-basalt  series  includes  igneous  rocks  with 
calcic  plagioclase,  olivine,  and  some  other  ferromagnesian  mineral 
as  the  essential  constituents.  They  contain  less  silica  than  the 
rocks  of  the  gabbro-auganite  series  on  account  of  the  presence  of 
olivine  { (Mg,Fe)2Si04;  Si02  =  about  40  per  cent.) . 

1  The  distinction  between  diorite  and  gabbro  is  based  upon  the  character  of  the  plagioclase 
rather  than  upon  the  ferromagnesian  mineral  for  it  is  known  that  in  many  eubsilicic  rocks 
hornblende  has  been  formed  from  pyroxene  at  a  late  magmatic  period. 


THE  ORIGIN  OF  MINERALS  435 

Olivine  Gabbro  is  a  grained  rock  consisting  essentially  of 
labradorite  (or  bytownite)  with  olivine  and  either  augite,  hypers- 
thene,  or  hornblende.  It  differs  from  gabbro  proper  in  the 
presence  of  olivine  and  should  have  a  distinctive  name  as  it 
belongs  to  a  distinct  series  and  is  not,  as  the  name  implies,  simply 
a  variety  of  gabbro. 

With  increase  in  the  amount  of  olivine  and  decrease  in  plagio- 
clase,  gabbros  grade  into  peridotites.  With  increase  of  the 
pyroxene  and  decrease  of  the  other  minerals  they  grade  into  the 
pyroxenites.  Pyroxene,  on  the  other  hand,  is  practically  absent  in 
some  varieties;  these  plagioclase-olivine  rocks  are  called  troctolites. 

Diabase.  This  name  is  used  for  a  common  rock  type  in  this 
series  which  is  characterized  by  a  peculiar  texture.  The  plagio- 
clase  occurs  in  lath-shaped  crystals  in  the  interstices  of  which 
the  augite  (and  olivine)  has  formed,  presumably  at  a  later 
period.  Ilmenite  is  a  characteristic  mineral  of  diabase  and  in 
some  diabases  analcite  occurs  as  an  original  mineral. 

The  diabases  usually  occur  in  the  smaller  intrusions  and  are 
thus  intermediate  in  mode  of  occurrence  between  the  subjacent 
gabbros  and  the  volcanic  basalts. 

Basalts  are  dense  igneous  rocks  with  calcic  plagioclase  (usually 
labradorite),  olivine,  and  either  augite  or  hornblende  as  essential 
minerals.  The  recognition  of  auganite  as  a  distinct  rock  type 
restricts  the  term  basalt  to  olivine-bearing  rocks.  The  texture 
of  basalt  usually  varies  from  felsitic  to  subporphyritic.  Pheno- 
crysts  are  rare  as  compared  with  those  in  corresponding  rocks 
in  the  other  series.  Vesicular  and  amygdaloidal  basalts  are 
common.  Zeolites  are  especially  characteristic,  but  calcite, 
quartz,  chalcedony,  epidote,  and  chlorite  are  also  frequently 
present.  Amygdaloidal  basalts  furnish  part  of  the  native  copper 
mined  in  the  Upper  Peninsula  of  Michigan. 

Basalts  are  as  a  rule  black  or  very  dark  gray  but  color  alone  is 
not  a  safe  distinction. 

The  basalts  usually  contain  some  interstitial  glass  mixed 
with  fine  magnetite  dust  and  thus  grade  into  basaltic  glasses, 


436          INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

which  are  often  found  as  thin  crusts  on  basalt  flows  or  as  narrow 
bands  on  the  sides  of  basalt  dikes. 

Nepheline  and  leucite  occur  in  some  rare  types  of  basaltic 
rocks  called  basanite. 

(g)  Peridotite-Limburgite  Series. 

The  peridotite-limburgite  series  includes  feldspar-free  igneous 
rocks  with  olivine  and  pyroxene  as  essential  constituents  with 
their  glassy  equivalents.  They  are  very  low  in  silica  and 
alumina  and  consequently  high  in  lime,  magnesia,  and  iron  oxids. 

Peridotites  are  grained  rocks  with  olivine  and  pyroxene,  but 
with  little  or  no  plagioclase.  They  grade  into  olivine  gabbros 
on  the  one  hand  and  into  nearly  pure  olivine  rocks  (dunites)  or 
nearly  pure  pyroxene  rocks  (pyroxenites)  on  the  other  hand. 
The  pyroxene  of  peridotite  may  be  augite,  hypersthene,  or 
enstatite,  often  an  intergrowth  of  two  of  them.  Hornblende  is 
sometimes  present.  Common  associated  minerals  are  mag- 
netite, ilmenite,  chromite,  and  pyrrhotite.  Garnet,  spinel,  and 
native  iron  are  also  found  in  peridotites. 

Peridotites  are  seldom  found  in  an  unaltered  condition  but 
their  alteration  products,  the  hydrothermal  metamorphic 
rocks  known  as  serpentine,  are  very  common.  The  principal 
mineral  of  serpentine  is  antigorite  formed  at  the  expense  of  the 
olivine  and  pyroxene  of  the  original  peridotite. 

The  peridotites  are  usually  found  in  dikes  rather  than  in  sub- 
jacent rock  masses.  This  is  one  reason  why  the  petrographic 
classification  of  igneous  rocks  should  be  based  upon  texture  rather 
than  upon  geological  occurrence. 

Limburgite,  the  dense  equivalent  of  peridotite,  is  a  very  rare 
rock  type  and  is  mentioned  here  simply  for  the  sake  of  complete- 
ness. It  has  been  called  magma-basalt.  But  the  term 
basalt  should  be  restricted  to  olivine-plagioclase-augite  rocks. 
The  limburgites  contain  phenocrysts  of  olivine  and  augite  in  a 
glassy  ground-mass.  Augitite  is  used  for  the  corresponding 
rock  with  augite  alone. 


THE  ORIGIN  OF  MINERALS  437 

OTHER  FELDSPAR-FREE  IGNEOUS  ROCKS 

Other  feldspar-free  rocks  include  some  rare  types  in  which 
the  feldspathoids,  nepheline  and  leucite,  take  the  place  of  the 
feldspars.  The  leucitites,  as  the  dense  leucite-pyroxene  rocks 
are  called,  are  abundant  in  Italy. 

There  are  also  rock  masses  with  large  amounts  of  magnetite 
and  ilmenite  which  are  often  considered  as  ultrabasic  igneous 
rocks.  A  rock  of  this  character  occurs  at  Cumberland  Hill, 
Rhode  Island.  It  contains  magnetite,  ilmenite,  olivine,  and 
plagioclase.  The  silica  content  is  only  22  per  cent.,  while  the 
combined  iron  oxids  reach  as  high  as  43  per  cent. 

2.  VOLCANIC  EMANATIONS 

It  is  a  well-known  fact  that  volcanoes  emit  gases  along  with 
lava  and  fragmental  products.  Of  these  gases  the  principal 
one  is  water  vapor  or  steam.  Others  include  hydrogen,  oxygen, 
nitrogen,  hydrogen  sulfid,  sulfur  dioxid,  carbon  dioxid,  hydro- 
chloric acid,  etc.  The  openings  through  which  the  gases  issue 
are  called  fumaroles. 

In  contrast  with  the  prominence  of  dissolved  gases  in  deep- 
seated  igneous  rocks,  the  gases  in  volcanic  rocks  escape  and  so  as 
a  rule  they  play  a  comparatively  small  role. 

Under  favorable  conditions,  however,  volcanic  sublimates  may 
form.  The  principal  minerals  that  appear  as  volcanic  sublimates 
are:  sulfur,  sal-ammoniac  (NH4C1),  halite,  sylvite,  hematite, 
tenorite  (crystalline  CuO),  covellite,  sassolite  (H3B03),  gypsum, 
and  hornblende. 

It  is  probable  that  the  high-temperature  forms  of  silica,  tridymite 
and  cristobalite,  are  formed  by  volcanic  gases. 

3.  PEGMATITES 

Closely  related  to  plutonic  igneous  rocks  and  grading  into  the 
high-temperature  veins  are  the  rocks  known  as  pegmatites. 
They  occur  in  relatively  narrow  dikes  or  veins  and  have  been 
formed  during  the  last  stages  in  the  consolidation  of  subjacent 


438        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

rocks.  They  are  supposed  to  be  due  to  the  crystallization  of  a 
sort  of  mother  liquor  left  after  the  main  rock  has  consolidated. 
Mineralizers  or  dissolved  gases  probably  account  for  many  of 
the  peculiar  features  of  pegmatites.  They  are  coarse  grained  and 
are  often  characterized  by  the  presence  of  very  large  crystals. 
Immense  muscovite,  feldspar,  beryl,  and  spodumene  crystals 
sometimes  occur  in  them.  At  one  locality  in  the  Black  Hills, 
South  Dakota,  spodumene  crystals  up  to  a  length  of  14  meters 
were  found.  These  are  the  largest  crystals  on  record.  Other 
characteristics  of  pegmatites  are  the  simultaneous  crystalliza- 
tion of  two  or  more  minerals  such  as  graphic  granite  (quartz 
and  feldspar)  and  perthite  (microcline  and  albite),  the  varia- 
bility in  texture  in  different  parts  of  the  vein,  and  the  presence  of 
rare  elements  and  rare  minerals. 

There  are  pegmatites  corresponding  to  most  of  the  known 
grained  rocks  but  the  granite  pegmatites  are  especially  common 
and  the  term  pegmatite  without  any  qualification  usually  refers 
to  granite  pegmatites.  They  are  very  prominent  in  New  Eng- 
land, South  Dakota,  and  southern  California.  The  following 
minerals  are  found  in  granite  pegmatites :  quartz  (both  a-quartz 
and  /3-quartz,  so  that  the  temperature  of  formation  was  probably 
in  the  neighborhood  of  575°C.),  microcline  (the  common  feldspar 
of  pegmatites),  orthoclase,  albite,  muscovite,  lepidolite,  tourma- 
line, amblygonite  [(Li,Al)(F,OH)PO4],  topaz,  beryl,  apatite, 
fluorite,  columbite,  garnet,  andalusite,  and  also  many  rare 
minerals  containing  such  elements  as  caesium,  zirconium,  uran- 
ium, tantalum,  etc. 

In  southern  Norway  over  70  minerals,  most  of  them  very  rare, 
have  been  found  in  the  syenite  and  nepheline  syenite  pegmatites 
of  that  region. 

4.  PYROCLASTIC  ROCKS 

The  pyroclastic  rocks  are  the  fragmental  products  of  igneous 
activity  and  their  consolidated  equivalents.  They  form  a  con- 
necting link  between  volcanic  igneous  rocks,  with  which  they  are 


THE  ORIGIN  OF  MINERALS  439 

usually  associated,  and  sedimentary  rocks  of  mechanical  origin. 
A  volcanic  ash  worked  over  by  water  may  form  a  tuffaceous 
sandstone. 

The  pyroclasts  are  formed  during  the  explosive  activity  of 
volcanoes  and  thus  these  materials  are  often  found  interbedded 
with  lavas.  The  fragments  may  vary  in  size  from  very  large 
volcanic  bombs  through  lapilli  (from  a  few  cms.  to  a  few  mms.  in 
size)  and  so-called  volcanic  ash  down  to  the  extremely  fine  ma- 
terial known  as  volcanic  dust,  so  fine  that  it  floats  in  the 
atmosphere  for  hours  or  even  days  before  it  settles. 

Consolidated  volcanic  ash  is  called  tuff  and  consolidated  rock 
of  coarser  fragments,  agglomerate.  Tuffs  and  agglomerates  are 
common  in  volcanic  regions.  The  older  tuffs  are  firmly  con- 
solidated and  on  account  of  alteration  often  may  be  dis- 
tinguished only  with  difficulty. 

Except  for  their  fragmental  character  pyroclasts  resemble 
dense  igneous  rocks.  Many  of  them,  however,  consist  largely 
of  glass,  often  the  pumice  variety.  Phenocrysts  are  common  and 
rock  fragments,  i.e.,  composite  fragments  of  two  or  more  minerals 
or  minerals  and  glass,  are  very  characteristic. 

In  chemical  composition  and  mineral  components  the  pyro- 
clastic  rocks  correspond  to  any  of  the  volcanic  or  dense  rocks. 
Thus  we  have  rhyolitic  tuff,  andesitic  tuff,  etc. 

5.    SEDIMENTARY  ROCKS 

Sedimentary  rock  is  a  convenient  collective  term  for  stratified 
rock  laid  down  for  the  most  part  under  water.  In  some  of  the 
sedimentary  rocks  wind  action  may  have  been  prominent.  The 
sedimentary  rocks  may  be  grouped  under  three  main  heads: 
those  of  (1)  mechanical  origin,  (2)  organic  origin,  and  (3)  chemical 
origin. 

(a)  Sedimentary  Rocks  of  Mechanical  Origin. 

These  rocks  are  usually  stratified.  They  are  derived  from 
pre-existing  rocks  by  mechanical  agents.  Their  concentration 


440        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

has  been  preceded  by  weathering,  a  term  used  for  superficial 
changes  brought  about  by  atmospheric  agents  and  meteoric 
water.  The  agents  effective  in  weathering  are  in  part  chemical 
and  in  part  physical.  The  chemical  changes  involved  are  mainly 
oxidation,  carbonation,  and  hydra tion.  Comparatively  few 
minerals  are  produced  by  the  weathering  of  the  ordinary  rock- 
forming  silicates.  The  principal  ones  are  limonite,  turyite, 
cliachite,  gibbsite,  kaolinite,  and  halloysite.  The  tendency  of 
weathering  is  toward  simplification.  Most  of  the  minerals 
present  in  weathered  rocks  are  residual  minerals  left  over  from 
an  early  stage.  The  importance  of  weathering  is  probably  over- 
emphasized, for  some  of  the  effects  ascribed  to  weathering  are  due 
to  hydrothermal  metamorphism.  Weathering  obviously  de- 
pends upon  climatic  conditions.  In  arid  climates  it  is  different 
from  that  in  humid  regions. 

Unconsolidated 

The  coarse  Unconsolidated  material,  gravel,  grades  through 
sand  into  silt.  The  minerals  of  sand  and  gravel  are  largely 
those  that  resist  wear  and  chemical  action.  For  this  reason 
quartz  is  undoubtedly  the  principal  mineral  of  sand  but  rarely 
does  the  sand  or  gravel  consist  entirely  of  quartz.  The  common 
rock-forming  minerals  such  as  feldspar  and  mica  are  also  found 
in  sands  and  in  favorable  places  where  there  has  been  natural 
concentration  we  have  heavy  minerals  such  as  magnetite, 
ilmenite,  garnet,  zircon,  rutile,  tourmaline,  and  occasionally 
diamond,  gold,  platinum,  and  cassiterite.  Locally  almost  any 
mineral  may  occur.  For  example,  on  the  Hawaiian  beaches 
olivine  is  the  only  common  mineral  of  the  sand.  In  the 
arid  regions  of  certain  parts  of  New  Mexico  and  Utah  the 
dune  sands  are  made  up  entirely  of  grains  of  gypsum.  On  the 
island  of  Ceylon  there  are  found  gem-bearing  gravels  with  such 
minerals  as  spinel,  corundum,  garnet,  zircon,  ilmenite,  monazite 
(Ce,La,Di)P04,  and  chrysoberyl  (BeAl204). 


THE  ORIGIN  OF  MINERALS  441 

Consolidated 

Breccia.  A  rock  made  up  of  coarse  angular  fragments  is 
known  as  a  breccia.  The  fragments  may  be  quartz,  chalcedony, 
calcite,  or  almost  any  mineral  or  even  rock  fragments.  The 
cementing  material  may  be  the  same  as,  or  different  from,  the 
fragments. 

Conglomerate.  Breccias  grade  into  conglomerates,  the  frag- 
ments of  which  are  sub-angular  to  rounded  and  are  due  to  me- 
chanical wear.  The  pebbles  of  conglomerates  usually  have  been 
transported  considerable  distances,  while  the  angular  fragments  of 
breccias  have  been  cemented  before  much  mechanical  action 
took  place. 

Sandstones.  Consolidated  sands  are  called  sandstones.  The 
grains  are  subangular  to  rounded.  The  principal  constituent  of 
sandstone  is  quartz,  but  other  minerals  such  as  feldspar,  musco- 
vite,  and  calcite  are  not  rare.  A  sandstone  with  appreciable 
amounts  of  feldspar  is  known  as  arkose.  Closely  related  to 
arkose  but  containing  in  addition  to  quartz  and  feldspar  miscel- 
laneous minerals  and  rock  fragments  such  as  bits  of  shale  is  the 
rock  known  as  graywacke.  It  is  a  kind  of  fine-grained  con- 
glomerate. According  to  the  character  of  the  cementing  material 
sandstones  are  classed  as  calcareous,  ferruginous,  siliceous,  or 
bituminous.  On  account  of  porosity  fossils  are  comparatively 
rare  in  sandstones.  The  calcite  in  calcareous  sandstones  is  due  in 
part  to  a  recrystallization  of  the  calcium  carbonate  of  calcareous 
fossils.  Loosely  cemented  sandstones  with  grains  of  glauconite 
are  called  greensands.  They  are  used  as  fertilizers  and  are  also 
a  possible  source  of  potassium  salts.  Greensands  are  prominent 
in  the  Cretaceous  of  New  Jersey. 

With  increasing  fineness  of  grain  sandstones  pass  into  shales. 
With  increase  in  the  amount  of  calcite  they  pass  into  limestone. 
There  are  also  gradations  between  sandstones  and  tuffs  (tuffa- 
ceous  sandstone)  and  between  sandstones  and  quartzites  (quartz- 
itic  sandstones). 


442        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Shales  are  more  or  less  laminated  sedimentary  rocks  made  up 
largely  of  indefinite  hydrous  aluminum  silicates.  They  may  also 
contain  quartz  and  calcite  and  thus  grade  on  the  one  hand  into 
sandstones  and  into  limestone  on  the  other.  Minerals  visible  to 
the  unaided  eye  include  pyrite  and  gypsum.  Shales  are  consoli- 
dated muds  and  clays  composed  for  the  most  part  of  the  finer 
materials  produced  by  land  waste.  Typical  shales  are  of  mechan- 
ical origin  but  some  of  the  siliceous  rocks  of  organic  origin  are 
also  known  as  shales  on  account  of  their  laminated  character. 

Clays  are  massive,  loosely  compacted  rocks,  containing  hydrous 
aluminum  silicates  (including  kaolinite  and  halloysite)  with  more 
or  less  finely  derived  quartz,  sericite,  and  sometimes  feldspar  and 
other  possible  minerals.  The  term  clay  is  often  restricted  to 
material  that  becomes  plastic  when  wet  with  water. 

Some  clays  are  derived  from  rocks  by  weathering  in  place. 
Besides  these  residual  clays  there  are  also  transported  clays. 

No  clear  distinction  can  be  drawn  between  clays  and  shales. 
Clays,  though,  are  mainly  terrestrial  deposits  and  shales  subma- 
rine deposits. 

Bauxite  is  a  surficial  rock  consisting  largely  of  amorphous 
cliachite  [A12O3(H2O)Z]  with  more  or  less  crystalline  gibbsite 
(Al2O3-3H2O).  It  usually  has  a  pisolitic  structure  but  some 
varieties  greatly  resemble  ordinary  clay.  Bauxite,  according  to 
recent  investigations,  is  formed  by  the  desilication  of  clay,  which 
in  turn  was  formed  from  such  igneous  rocks  as  syenite.  It  is 
found  in  Alabama,  Tennessee,  and  Arkansas,  and  probably  was 
formed  under  tropical  conditions  of  weathering. 

(&)  Sedimentary  Rocks  of  Organic  Origin. 

It  is  a  well-known  fact  that  certain  organisms  secrete  calcium 
carbonate  or  silica  from  the  waters  in  which  they  live  in  order  to 
build  up  hard  parts  (shells  or  tests)  for  the  support  of  their  soft 
tissues.  In  other  cases  the  organic  matter  of  the  organism  itself 
is  preserved  and  this  accumulated  material  may  form  a  rock. 

Limestone.     The   most   common   rock   of   organic   origin   is 


THE  ORIGIN  OF  MINERALS  443 

limestone,  which  consists  almost  entirely  of  the  mineral  calcite, 
usually  in  a  massive  form.  While  often  apparently  amorphous 
it  is  really  microcrystalline.  Calcium  carbonate  is  present  in  most 
natural  waters  and  is  secreted  by  various  organisms  such  as 
molluscs,  brachiopods,  echinoderms,  bryozoa,  corals,  certain 
sponges,  and  some  foraminifera.  The  organisms  just  named  are 
animals  but  some  plants  such  as  algse  also  secrete  calcium  car- 
bonate. Most  of  the  limestones  are  marine  in  origin  but  fresh 
water  limestones  are  also  known.  They  may  be  recognized  by 
the  presence  of  plant  stems  and  fresh  water  mollusc  shells  such 
as  those  of  snails. 

When  the  organisms  die,  the  organic  material  decays  and  there 
remain  the  shells  and  tests  and  other  hard  parts  such  as  crinoid 
stems  and  coral  fragments.  They  accumulate  and  are  more  or 
less  broken  up,  often  ground  to  bits  and  thus  form  a  calcareous 
mud  which  on  hardening  produces  a  limestone. 

Besides  the  mineral  calcite,  sedimentary  limestones  may  con- 
tain dolomite,  quartz,  chalcedony,  and  such  minerals  as  barite, 
celestite,  anhydrite,  gypsum,  siderite,  aragonite,  collophane,  and 
pyrite.  These  minerals  occur  for  the  most  part  in  seams  and 
cavities  along  with  recrystallized  calcite. 

Varieties  of  limestone  are  based  in  part  upon  the  character  of 
the  fossils  present,  e.g.,  crinoidal  limestone,  coral  limestone, 
mumrnulitic  limestone,  and  shell  limestone  (the  shell  limestone 
on  the  coast  of  Florida  is  called  coquina  and  represents  the  first 
stage  in  the  formation  of  some  limestone) .  Chalk  is  a  very  fine- 
grained porous  limestone  formed  by  the  consolidation  of  calcareous 
ooze  which  is  made  up  principally  of  tests  of  foraminifera. 
Though  chalk  is  apparently  amorphous,  microscopic  examination 
proves  it  to  be  microcrystalline. 

Depending  upon  the  impurities  present  such  varieties  of  lime- 
stone as  siliceous,  arenaceous,  argillaceous,  and  bituminous  are 
recognized.  These  show  gradations  toward  cherts,  sandstones, 
shales,  and  hydrocarbons  respectively.  A  limestone  that  will  take 
a  polish,  whether  sedimentary  or  metamorphic,  is  called  marble. 


444        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

The  name  limestone  is  not  restricted  to  sedimentary  rocks  of 
organic  origin  but  is  used  for  any  massive  rock  made  up  largely 
of  calcite.  Thus  we  have  oolitic  limestones  of  chemical  origin 
and  crystalline  limestones  of  metamorphic  origin. 

Diatomite  or  Diatomaceous  Earth.  Although  most  organisms 
secrete  calcium  carbonate  some  secrete  silica  in  the  form  of  opal. 
Among  animals  these  include  sponges  and  radiolaria  and  among 
plants,  diatoms.  Siliceous  deposits  are  forming  in  the  ocean  at 
the  present  time.  Thus  we  have  radiolarian  ooze  and  diatoma- 
ceous  ooze  produced  by  the  sinking  of  the  tests  of  radiolaria  and 
diatoms  from  near  the  surface  of  the  sea  to  the  bottom.  There 
are  also  calcareous  oozes  (globigerina  ooze)  and  argillaceous 
deposits  known  as  "red  clay." 

The  consolidation  of  these  oozes  after  the  uplift  of  the  sea 
furnishes  diatomite  or  diatomaceous  earth,  in  which  there  are 
found  not  only  diatoms  but  also  radiolaria  and  sponge  spicules, 
usually  in  a  fragmentary  condition.  These  deposits  are  mostly 
Tertiary  in  age  and  are  abundant  in  the  Western  United  States. 
A  very  large  deposit  is  now  being  quarried  near  Lompoc,  Santa 
Barbara  county,  California.  When  these  deposits  become  more 
or  less  laminated  and  mixed  with  detrital  material  they  are 
known  as  siliceous  shale.  It  is  possible  that  some  of  the  older 
cherts  are  of  organic  origin,  but  most  of  them  are  now  believed  to 
have  been  formed  by  the  replacement  of  limestones  or  shales  by 
silica. 

Carbonaceous  Rocks.  To  make  the  classification  of  the  or- 
ganically-derived rocks  complete  we  must  include  carbonaceous 
rocks  such  as  coal.  While  coal  is  not  a  mineral  it  may  be  con- 
sidered a  mineraloid  (see  page  413).  Coal  is  a  sedimentary  rock 
occurring  interbedded  with  shales  and  sandstones.  If  is  of 
plant  origin,  for  not  only  may  plant  tissues  be  recognized  in  thin 
sections  and  polished  surfaces,  but  an  almost  perfect  gradation 
may  be  traced  from  peat  through  lignite  to  the  various  types  of 
coal  such  as  sub-bituminous,  bituminous,  and  semi-bituminous 
into  semi-anthracite  and  anthracite.  Anthracite  may  be  con- 
sidered a  special  type  of  metamorphic  rock. 


THE  ORIGIN  OF  MINERALS  445 

Coal  consists  largely  of  three  elements:  carbon,  hydrogen,  and 
oxygen,  with  smaller  amounts  of  nitrogen  and  sulfur.  Analyses, 
however,  are  usually  given  in  the  form  of  proximate  constituents, 
which  include  moisture,  volatile  hydrocarbons,  fixed  carbon,  and 
ash.  Anthracite  is  highest  in  fixed  carbon  and  lowest  in  ash, 
while  lignite  is  lowest  in  fixed -carbon  and  highest  in  moisture. 

The  associated  minerals  of  coal  include  pyrite,  marcasite, 
gypsum,  dolomite,  melanterite  (FeSCVTH^O),  and  copiapite 
(basic  ferric  sulfate). 

Other  Deposits  of  Organic  Orgin. 

In  addition  to  calcite,  opal,  and  hydrocarbons,  certain  other 
minerals  are  believed  to  be  have  been  formed  at  times  by  organic 
agencies.  Among  them  are  sulfur  and  limonite. 

Cellophane  is  also  in  part  of  organic  origin.  The  bones  of 
vertebrate  animals  consist  of  calcium  carbonophosphate  to- 
gether with  an  organic  substance  called  ossein.  The  organic 
matter  is  gradually  eliminated  and  its  place  is  taken  by  the  cal- 
cium carbonophosphate  from  the  solution  of  other  bones.  Some- 
times bones  accumulate  and  form  bone-breccias.  The  cementing 
material  of  the  bone  fragments  is  calcite  and  there  may  be 
partial  replacement  of  the  collophane  by  calcite. 

Most  of  the  phosphorites  or  so-called  phosphate  rocks  seem  to 
have  been  formed  by  the  replacement  of  limestones  by  collophane. 

(c)  Sedimentary  Rocks  of  Chemical  Origin. 

In  this  division  are  placed  those  rocks  which  have  been  formed 
by  the  evaporation  of  inland  bodies  of  water  or  those  that  have 
been  formed  by  the  replacement  of  mechanical  or  organic  sedi- 
ments by  solutions.  Replacement  deposits  of  hydrothermal 
origin  are  discussed  under  metamorphic  rocks.  Rocks  of  this 
group  are  perhaps  not  as  common  as  the  two  preceding  divisions 
but  they  are  fully  as  important  for  they  include  many  deposits 
of  economic  value. 

Gypsum  is  a  rock  as  well  as  a  mineral.  It  occurs  in  thick 
massive  beds  associated  with  shales  and  limestones  and  owes  its 


446        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

origin  in  many  cases  to  the  evaporation  of  inland  seas.  Hence 
it  is  often  associated  with  saline  deposits.  Among  the  minerals 
it  may  contain  are  calcite,  dolomite,  celestite,  sulfur,  and  quartz. 

Anhydrite,  both  as  a  mineral  and  as  a  rock,  is  a  frequent  asso- 
ciate of  gypsum,  and  it  has  been  proved  that  some  gypsum  de- 
posits have  been  formed  by  the  hydration  of  anhydrite. 

Secondary  deposits  of  gypsum  in  an  impure  and  loosely  com- 
pacted form  are  known  as  gypsite  or  gypsum  earth. 

Gypsum  as  a  rock  is  found  in  New  York,  Michigan,  Iowa, 
Kansas,  Oklahoma,  New  Mexico,  and  other  states. 

Anhydrite  like  gypsum  occurs  as  a  rock  as  well  as  a  mineral. 
It  occurs  in  beds  and  even  more  frequently  than  gypsum  it  is 
associated  with  saline  deposits.  The  reason  for  this  is  as  follows : 
sea-water  on  evaporation  first  yields  hydrous  calcium  sulfate 
(gypsum),  then  when  the  solution  becomes  sufficiently  high  in 
sodium  chlorid,  anhydrous  calcium  sulfate  (anhydrite)  forms  and 
soon  afterwards  sodium  chlorid  (halite)  is  deposited;  so  that  in 
many  salt  mines  an  anhydrite  bed  is  found  directly  below  the 
bed  of  rock-salt. 

The  minerals  associated  with  anhydrite  include,  besides  halite, 
dolomite,  calcite,  quartz,  and  gypsum.  Near  the  surface  anhy- 
drite is  usually  converted  into  gypsum  and  many  gypsum  beds 
have  thus  been  formed. 

Anhydrite  rocks  are  found  in  Michigan,  Kansas,  Oklahoma, 
Texas,  and  Nevada.  It  is  a  prominent  rock  in  the  Stassfurt 
(Prussia)  salt  deposits. 

Rock-salt.  Halite  or  sodium  chlorid  often  occurs  in  thick  beds 
and  hence  must  be  classed  as  a  rock.  It  is  formed  by  the  evap- 
oration of  sea-water  in  enclosed  bays  or  inland  seas  after  calcium 
sulfate  has  been  deposited.  Overlying  shales  or  clays  are  neces- 
sary to  protect  the  bed  of  rock-salt  from  solution  unless  the 
region  is  an  excessively  arid  one.  Anhydrite  is  probably  the 
most  commonly  associated  mineral  but  the  chlorids  and  sulfates 
of  potassium  and  magnesium  also  occur.  Of  these  the  most 
important  is  sylvite  (KC1).  Strange  to  say,  halite  and  sylvite 


THE  ORIGIN  OF  MINERALS  447 

occur  side  by  side  and  do  not  form  an  isomorphous  mixture  as 
one  would  expect. 

Rock-salt  is  found  in  nearly  all  parts  of  the  world  and  in  beds 
that  were  deposited  in  practically  all  the  geological  periods. 

Other  Saline  Deposits.  Sea-water  also  contains  potassium  and 
magnesium  and  when  the  evaporation  is  complete  salts  of  these 
metals  will  crystallize  out.  This  has  happened  in  a  few  cases  and 
the  deposits  thus  formed  have  been  protected  from  solution  by 
overlying  clays. 

The  important  minerals  of  these  deposits  are:  sylvite  (KC1), 
carnallite  (KMgCl3-6H2O),  kainite  (KMgCl(SO4)-3H2O),  kieser- 
ite  (MgSO4-H20),  and  polyhalite  (K2MgCa2(SO4)4-2H2O)  in 
addition  to  halite.  Of  these  kainite  and  sylvite  are  said  to  be 
secondary  minerals,  that  is  they  have  been  formed  at  the  expense 
of  the  others  (primary)  after  they  were  deposited  in  beds. 

At  Stassfurt,  Prussia,  the  deposits  of  the  above-mentioned 
minerals  are  of  great  commercial  importance,  as  are  also  the 
more  recently  discovered  potash  deposits  near  Mulhouse,  Alsace, 
where  sylvite  and  halite  occur  together  in  immense  beds.  An- 
other commercial  deposit  is  at  Suria  in  Catalonia  (Spain),  where 
sylvite  and  carnallite  occur  in  deposits  of  Tertiary  age. 

Soda  Lake  Deposits.  Although  these  deposits  are  not  usually 
considered  rocks  they  deserve  notice  at  this  place.  Sodium  car- 
bonate, sodium  borate,  and  sodium  sulfate  as  well  as  sodium 
chlorid  occur  as  the  result  of  the  evaporation  of  lakes  in  arid 
regions.  These  lake  deposits  are  found  in  Wyoming,  Nevada, 
and  California.  The  principal  minerals  are  trona  (Na2CO3-Na- 
HCO3-2H2O),  mirabilite  (Na2SO4-10H2O),  thenardite  (Na2SO4), 
gaylussite  (Na2CO3-CaCO3-5H2O),  pirssonite  (Na2CO3-CaCO3- 
2H2O),  epsomite  (MgSO4-7H2O),  glauberite  (Na2SO4-CaSO4), 
hanksite  (9Na2SO4-2Na2CO3-KCl),  borax  (Na2B407-10H2O),  and 
ulexite  (NaCaBO59-8H2O). 

Nitrate  Deposits.  Extensive  deposits  of  nitratine  (NaNO3) 
are  found  in  the  arid  regions  of  northern  Chile  in  superficial 
beds  up  to  two  meters  in  thickness.  The  associated  minerals 
are  halite,  gypsum,  and  lautarite  [Ca(IO3)2]. 


448        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Oolitic  limestone.  A  variety  of  limestone  made  up  of 
minute  spherical  concretions  resembling  fish  roe  and  known  as 
oolitic  limestone  is  common  in  the  Pennsylvanian  of  the  Middle 
West  and  in  the  Jurassic  of  England.  Along  the  shores  of  the 
Great  Salt  Lake  of  Utah  an  oolitic  sand  now  forming  evidently 
represents  the  first  stage  in  the  production  of  oolitic  limestone. 

Most  other  oolitic  rocks  seem  to  have  been  derived  from 
oolitic  limestones  by  replacing  solutions,  but  the  oolitic  phos- 
phorite of  southeastern  Idaho  is  probably  a  chemically  formed 
deposit  formed  by  the  direct  deposition  of  cellophane. 

Travertine.  Calcium  carbonate  is  soluble  in  carbonated 
water  and  on  the  escape  of  the  carbon  dioxid  due  to  release 
of  pressure,  calcium  carbonate  is  deposited  in  the  form  of  calcite 
or  more  rarely  in  the  form  of  aragonite.  It  is  this  process  that 
accounts  for  certain  calcareous  deposits,  the  banded  ones  of 
which  are  called  travertine.  Some  of  these  are  spring  deposits 
and  some  are  cave  deposits  closely  related  to  the  familiar  stalac- 
tites and  stalagmites. 

Calcareous  tufa.  A  more  or  less  porous  calcareous  deposit 
formed  by  springs  is  known  as  calcareous  tufa.  (Tufa  should 
be  distinguished  from  tuff,  a  fragmental  volcanic  rock).  The 
calcareous  tufas  often  show  remains  of  mosses.  Aquatic  plants 
like  terrestial  ones  require  carbon  dioxid  for  their  existence  and 
so  when  the  carbon  dioxid  is  extracted  from  the  water  by  the 
plant  the  calcium  carbonate  is  precipitated.  The  deposit  then  is 
in  part  of  indirect  organic  organ.  This  example  shows  the 
impossibility  of  establishing  a  satisfactory  classification  of 
rocks  for  there  is  no  exact  dividing  line  between  the  organic 
and  chemical  deposits.  Some  of  the  calcareous  tufas  are  lake 
deposits,  for  example,  those  in  the  Great  Basin  region  of  Nevada. 

Other  limestones  of  organic  origin.  All  limestones  are 
not  of  organic  origin.  It  seems  probable  that  calcium  carbonate 
may  be  deposited  from  solution  under  certain  conditions  not 
including  those  mentioned  above. 

Thus  the  limestone  occurring  at  Green  River,  Wyoming,  was 


THE  ORIGIN  OF  MINERALS  449 

formed  in  brackish  water.  This  is  known  from  the  character  of 
the  fossil  fish  present. 

Siliceous  sinter.  Silica  in  the  form  of  opal  is  deposited  from 
hot  springs,  as  for  example,  Yellowstone  National  Park.  The 
rock  is  called  geyserite.  Algae  living  in  the  hot  water  secrete 
silica  and  thus  the  deposits  grade  into  the  organically-formed 
rocks. 

Ice.  To  be  complete,  ice  should  be  mentioned  here.  Ice  is 
a  rock  as  well  as  a  mineral  of  especial  prominence  in  the  polar 
regions. 

Rocks  formed  by  replacement  of  limestone.  Most  of  the 
sedimentary  rocks  made  up  of  dolomite,  chalcedony,  and  collo- 
phane  seem  to  have  been  formed  by  the  replacement  of  limestone. 
These  rocks,  however,  can  hardly  be  called  metamorphic  for  the 
replacement  was  brought  about  by  sea-water  or  by  percolating 
meteoric  waters  and  not  by  ascending  hydrothermal  solutions. 

Dolomitic  limestone  is  a  sedimentary  rock  made  up  largely 
of  the  mineral  dolomite  with  more  or  less  calcite.  There  is  good 
evidence  to  show  that  the  majority  of  dolomitic  limestones  were 
formed  from  ordinary  limestones  while  they  were  still  under 
the  sea.  This  process,  which  consists  in  the  removal  of  part  of 
the  calcium  and  the  substitution  of  magnesium  in  its  place,  is 
called  dolomitization.  But  in  some  cases  dolomitic  limestones 
have  been  formed  by  the  replacement  of  limestone  after  it  was 
.uplifted  above  the  sea,  sometimes  by  the  agency  of  meteoric 
waters  and  sometimes  by  hydrothermal  solution  in  connection  with 
ore  deposition. 

Chert.  Most  of  the  compact  massive  rocks  made  up  largely 
of  chalcedony  and  known  as  chert  or  flint  were  probably  formed 
by  the  replacement  of  limestones  or  shales  by  means  of  siliceous 
solutions.  Although  radiolaria,  diatoms,  and  sponge  spicules 
doubtless  have  contributed  their  share  of  silica,  most  of  the 
fossils  present  in  cherts  were  originally  calcareous.  Some  of 
the  cherts,  however,  may  have  been  formed  from  radiolarian 
ooze. 

29 


450        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Phosphorite  or  so-called  phosphate  rock  is  a  sedimentary  rock 
composed  largely  of  the  amorphous  mineral  cellophane,  a  cal- 
cium carbonate-phosphate.  The  associated  minerals  include 
dahllite  (the  crystalline  equivalent  of  cellophane),  calcite, fluorite, 
gypsum,  chalcedony,  and  quartz.  Although  the  phosphatic  mate- 
rial is  in  part  of  organic  origin,  most  of  the  phosphorites  seem  to  be 
replacement  of  limestones  as  fossils  present  were  originally 
calcareous. 

The  high-grade  phosphorites  are  used  extensively  as  a  source 
of  calcium  superphosphate,  which  is  employed  as  a  fertilizer. 
They  occur  in  Florida,  Tennessee,  and  also  in  southeastern 
Idaho  and  the  adjoining  portions  of  Wyoming  and  Utah. 

6.  METAMORPHIC  ROCKS 

The  third  great  group  of  rocks  consists  of  those  which  though 
originally  either  igneous  or  sedimentary  now  possess  characters 
that  entitle  them  to  recognition  as  a  separate  group.  New 
minerals  or  new  textures  or  both  have  been  developed  by  geologic 
agents  such  as  heat,  pressure,  or  hydrothermal  solutions.  The 
term  metamorphism,  though  sometimes  used  in  a  broad  sense 
for  practically  all  rocks  except  the  igneous,  is  generally  restricted 
to  changes  brought  about  by  hypogene  or  deep-seated  forces  acting 
from  within  the  earth's  crust  or  outer  shell.  Used  in  this  sense 
metamorphism  excludes  weathering  and  the  replacement  of 
sedimentary  rocks  by  solutions  of  meteoric  water. 

A  number  of  minerals  such  as  antigorite,  talc,  chlorite,  tremo- 
lite,  kyanite,  sillimanite,  staurolite,  graphite,  diopside,  forsterite, 
chondrodite,  phlogopite,  vesuvianite,  glaucophane,  and  wollasto- 
nite  are  highly  characteristic  of  metamorphic  rocks. 
Most  of  the  metamorphic  rocks  have  a  foliated  or  laminated 
structure  but  some  of  them  lack  this  and  are  massive. 

No  satisfactory  classification  of  the  metamorphic  rocks  has 
yet  been  devised.  Some  of  the  names  used  are  based  upon  chem- 
ical composition  and  some  upon  structure.  They  may  be  dis- 
cussed conveniently  under  three  great  heads:  rocks  produced  by 


THE  ORIGIN  OF  MINERALS  451 

regional,  contact,  and  hydrothermal  met  amor  phism,  the  chief 
agents  of  which  are  pressure,  heat,  and  hot  solutions  respectively. 

(a)  Regional  Metamorphism. 

Gneiss  is  a  coarsely  laminated  metamorphic  rock  of  a  great 
variety  of  mineral  components.  The  term  usually  is  used  in  a 
purely  structural  or  textural  sense.  Most  gneisses  are  recrystal- 
lized  igneous  rocks  and  hence  feldspars  are  usually  present. 
Other  minerals  include  quartz,  muscovite,  biotite,  muscovite, 
hornblende,  glaucophane,  epidote,  chlorite,  sillimanite,  and 
garnet. 

Varietal  names  for  gneisses  are  derived  (1)  from  a  prominent 
mineral  present,  e.g.,  biotite  gneiss,  and  (2)  from  the  original  rock 
that  produced  the  gneiss,  e.g.,  granite  gneiss.  Some  of  the  gneisses 
are  derived  from  sedimentary  rocks  such  as  conglomerates.  The 
chemical  analysis  may  sometimes  be  used  to  distinguish  a 
sedimentary  gneiss  from  an  igneous  gneiss. 

No  exact  dividing  line  can  be  drawn  between  gneiss  proper  and 
the  banded  gneissoid  igneous  rocks  in  which  the  banding  is 
believed  to  be  original.  With  increasing  fineness  of  texture  the 
gneisses  grade  into  schists. 

Schists  are  finely  laminated  metamorphic  rocks  intermediate 
in  texture  between  the  gneisses  and  the  slates  and  of  almost 
any  composition.  Varietal  names  are  based  upon  prominent 
minerals  present.  Thus  we  recognize  mica  schist,  hornblende 
schist,  talc  schist,  graphite  schist,  etc.  The  micas  (muscovite, 
biotite,  and  sericite)  are  probably  the  most  conspicuous  minerals. 
Feldspars  are  usually  absent  and  in  cases  of  doubt  their  presence 
or  absence  may  help  to  decide  whether  to  call  the  rock  gneiss  or 
schist.  Garnet  is  a  common  mineral  and  such  minerals  as 
staurolite,  kyaniter  andalusite,  sillimanite,  epidote,  glaucophane, 
and  tourmaline  are  frequently  found. 

Most  of  the  common  varieties  of.  schists  have  been  formed  from 
sedimentary  rocks  but  some  of  them  were  originally  igneous  rocks. 

Slates.     The  mica  schists  grade  into  the  slates,  which  are 


452        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

argillaceous  metamorphic  rocks  characterized  by  extremely  well 
developed  rock  cleavage.  The  material  of  slate  seems  to  be 
largely  sericite  in  very  minute  scales,  but  visible  minerals  include 
pyrite,  calcite,  and  dolomite.  The  coloring  matter  is  usually 
carbonaceous  matter  (black) ,  chlorite  (green) ,  or  iron  oxide  (red) . 

The  slates  with  an  even,  persistent  rock  cleavage  are  used 
for  roofing  purposes,  but  the  great  majority  of  slates  have  no 
economic  value. 

Quartzites  are  the  metamorphic  equivalents  of  sandstones  and 
may  be  distinguished  from  the  latter  by  the  fact  that  the  fracture 
takes  place  through  quartz  grains  and  cement  alike,  so  firmly  has 
the  rock  been  cemented.  Quartz  and  chalcedony  are  the  princi- 
pal minerals  but  feldspars,  sericite,  chlorite,  kyanite,  and  other 
silicates  are  sometimes  present.  The  quartz  often  shows  secon- 
dary enlargement  of  the  original  sand  grains  when  examined  in 
thin  sections.  The  later  deposited  quartz  then  appears  in  optical 
continuity  with  the  original  grains  of  quartz. 

Quartzites  grade  through  schistose  quartzites  into  quartz 
schists  and  through  quartzitic  sandstones  into  ordinary  sand- 
stones. 

Some  of  the  quartzites  are  simply  local  occurrences  of  hard- 
ened sandstones  and  are  not,  properly  speaking,  metamorphic 
rocks.  This  is  another  example  of  the  unsatisfactory  nature  of 
rock  classification. 

Quartzites  are  used  for  paving  blocks  and  for  the  manufacture 
of  silica  bricks. 

Crystalline  limestones  are  metamorphosed  sedimentary  lime- 
stones The  term  crystalline  is  used  in  an  arbitrary  way,  for  all 
limestones  are  made  up  of  crystalline  calcite.  The  crystalline 
limestones  are  sometimes  called  marble  but  the  latter  term  is 
preferably  used  for  any  limestone  that  will  take  a  polish  and 
can  be  used  commercially,  whether  it  is  sedimentary  or  meta- 
morphic. 

If  the  original  sedimentary  limestone  were  pure  the  result  of 
metamorphism  is  simply  recrystallization  and  the  resulting 


THE  ORIGIN  OF  MINERALS  453 

product  may  be  fine-,  medium-,  or  coarse-grained.  Schistose 
structures  are  rarely  developed  in  limestone  on  account  of  the 
peculiar  behavior  of  calcite  under  pressure.  Under  pressure 
calcite  crystals  undergo  gliding,  which  is  a  slipping  of  adjacent 
layers  on  each  other  along  crystallographic  directions.  In 
calcite  the  gliding  plane  is  the  negative  rhombohedron  e  {0112}. 
The  gliding  relieves  the  strains  and  there  is  little  or  no  parallel 
arrangement  of  the  calcite  grains.  In  case  the  original  limestone 
contained  impurities  such  as  clay  and  sand,  recystallization 
develops  such  silicate  minerals  as  diopside,  tremolite,  garnet, 
vesuvianite,  etc.  Graphite  is  also  very  common  and  is  formed 
from  organic  matter. 

Crystalline  dolomitic  limestones  are  formed  by  the  recrystal- 
lization  of  sedimentary  dolomitic  limestones.  They  consist 
largely  of  dolomite  and  calcite.  Other  minerals  present  are 
graphite,  spinel,  phlogopite,  forsterite,  cnondrodite,  etc.  The 
rocks  that  contain  the  magnesium  silicates  in  appreciable 
amounts  contain  calcite  and  little  or  no  dolomite.  During 
metamorphism  the  magnesium  seems  to  have  a  greater  affinity 
for  silica  and  alumina  than  calcium  has.  This  process  is  called 
dedolomitization,  as  it  is  in  some  respects  the  reverse  of 
dolomitization. 

(6)  Contact    Metamorphism. 

When  an  igneous  intrusion  breaks  through  sedimentary  lime- 
stone there  is  often  developed  a  contact  zone  with  silicate 
minerals.  The  minerals  in  the  contact  zone  are  due  partly  to 
recrystallization  t)f  the  impurities  of  the  original  limestone  but 
there  has  often  been  an  addition  of  material  from  the  igneous 
intrusion  itself.  For  example,  andradite,  the  calcium-iron  gar- 
net, is  a  common  mineral  in  these  contact  zones  and  it  is  prac- 
tically certain  that  the  iron  at  least  is  derived  from  the  intrusive 
magma  as  no  sedimentary  limestone  contains  such  large  amounts 
of  iron  without  a  correspondingly  large  amount  of  aluminum. 

The  common  minerals  of  the  contact  zone  include  andradite, 


454         INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

grossularite,  wollastonite,  vesuvianite,  diopside,  tremolite,  scapo- 
lite,  epidote,  tourmaline,  f  orsterite,  and  rare  minerals  too  numerous 
to  mention,  some  of  which  are  highly  characteristic.  The  contact 
metamorphic  zones  rival  the  granite  pegmatites  in  number  and 
variety  of  minerals  present. 

In  these  contact  zones  ore-deposits  are  often  encountered. 
The  prominent  ore-minerals  are  magnetite,  hematite,  pyrite, 
pyrrhotite,  chalcopyrite,  bornite,  sphalerite,  and  molybdenite. 
The  abundance  of  these  minerals  in  places  also  points  to  the 
probability  that  the  magma  furnished  most  of  the  iron,  copper, 
sulfur,  etc.,  necessary  for  the  production  of  these  minerals.  At 
a  stage  later  than  the  contact  metamorphism  the  silicate  minerals 
mentioned  above  may  be  converted  into  such  minerals  as  anti- 
gorite,  talc,  tremolite,  chlorite,  and  sericite  and  the  ore-minerals 
changed  to  chalcocite  and  covellite. 

By  contact  metamorphism  argillaceous  rocks  such  as  shales 
may  be  converted  into  slates  or  into  the  fine-grained  metamorphic 
rock  known  as  hornfels.  The  prominent  minerals  of  hornfels  are 
andalusite,  garnet,  kyanite,  tourmaline,  and  cordierite. 

(c)  Hydrothermal  Metamorphism. 

This  is  the  type  of  metamorphism  caused  by  hot  ascending 
solutions,  which  are  usually  alkaline.  Some  of  the  mineral  aggre- 
gates ordinarily  called  rocks,  such  as  the  serpentines  for  example, 
are  formed  by  hydrothermal  metamorphism  but  most  of  the 
hydrothermal  products  are  found  in  the  altered  zones  adjacent  to 
ore-bearing  veins.  Although  these  altered  zones  are  not  usually 
considered  rocks  they  deserve  some  attention  at  this  point. 

The  following  minerals  are  characteristic  of  hydrothermal 
metamorphic  rocks  and  zones:  quartz,  chalcedony,  sericite, 
adularia,  albite,  chlorite,  pyrophyllite,  antigorite,  talc,  calcite, 
dolomite,  siderite,  fluorite,  barite,  tourmaline,  biotite,  magnetite, 
muscovite,  epidote,  tremolite,  alunite,  topaz,  and  tourmaline. 
Of  these,  tourmaline,  muscovite,  and  topaz  are  formed  at  fairly 
high  temperatures  and  others  such  as  chalcedony  and  adularia 


THE  ORIGIN  OF  MINERALS  455 

and  probably  sericite,  tremolite,  antigorite,  and  talc  are  fairly  low- 
temperature  minerals.  Associated  with  the  minerals  above 
enumerated  we  find  in  greater  or  less  amount  the  following 
common  ore-minerals :  pyrite,  chalcopyrite,  sphalerite,  and  galena. 

Serpentine.  Although  hydrothermal  processes  may  play  a 
minor  part  in  the  formation  of  most  of  the  metamorphic  rocks,  at 
least  one  rock  type,  namely,  serpentine,  must  be  placed  in  the 
hydrothermal  division.  Serpentine  is  here  used  as  the  name  of  a 
massive  metamorphic  rock  consisting  essentially  of  the  mineral 
antigorite.  The  minerals  associated  with  antigorite  include 
chrysotile,  magnetite,  chromite,  olivine,  and  pyroxene.  Of  these 
the  last  three  are  residual  minerals  from  the  original  rock.  The 
great  majority  of  serpentines  were  derived  from  peridotites 
and  related  rocks  such  as  pyroxenites  and  dunites.  The  altera- 
tion is  largely  one  of  hydration  but  the  process  is  not  due  to 
weathering,  for  the  serpentine  undergoes  still  further  changes 
when  exposed  to  the  surface.  Talc  is  often  associated  with  anti- 
gorite and  in  some  cases  at  least  it  has  been  derived  from 
serpentine  by  a  still  further  hydrothermal  alteration. 

Ophicalcite  is  a  transitional  rock  between  crystalline  limestone 
and  serpentine.  The  principal  minerals  are  calcite  and  antigor- 
ite; the  antigorite  has  been  formed  from  forsterite  or  diopside 
of  a  crystalline  limestone  by  hydrothermal  metamorphism. 

Verde-antique  is  a  related  serpentinous  rock  with  white  veins 
of  calcite,  dolomite,  or  magnesite. 

7.  VEINS  AND  REPLACEMENT  DEPOSITS 

Under  favorable  conditions  of  temperature  and  hydrostatic 
pressure  practically  all  inorganic  substances  are  soluble  in 
certain  kinds  of  water.  Carbonates  are  soluble  in  carbonated 
water;  silica  is  soluble  in  hot  water  containing  alkaline  car- 
bonates. Veins  are  formed  by  the  deposition  of  these  or  other 
soluble  materials  along  relatively  narrow  fissures,  usually  by 
ascending  solutions.  In  addition  to  these  fissure  veins,  there 
are  also  replacement  veins  formed  by  the  replacement  of  the 


456        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

country  rock  along  more  or  less  definite  zones.  The  replacement 
veins  grade  into  irregular  replacement  deposits.  These  are 
especially  common  in  limestones. 

The  solutions  that  produce  the  greater  number  of  ore-bearing 
veins  are  believed  to  have  their  origin  in  a  magma,  for  it  is  a 
well-known  fact  that  outside  the  iron  ores  and  some  of  the  lead 
and  zinc  ores  most  ore-deposits  are  found  in  regions  of  igneous 
rocks.  It  is  also  significant  that  the  associated  silicate  minerals 
of  veins  and  replacement  deposits  are  almost  exclusively  those 
that  are  found  in  altered  igneous  rocks.  No  clear  distinction 
can  be  drawn  between  replacement  deposits  and  hydrothermal 
metamorphic  deposits. 

The  principal  gangue  minerals  of  veins  and  replacement  de- 
posits are:  quartz  (a-quartz),  chalcedony,  calcite,  dolomite,  siderite, 
rhodochrosite,  barite,  fluorite,  adularia,  albite,  tourmaline,  and 
alunite,  and  more  rarely  anhydrite,  lepidolite,  rhodonite,  and  the 
zeolites. 

The  chief  ore-minerals  associated  with  these  are  pyrite,  pyr- 
rhotite,  marcasite,  galena,  sphalerite,  chalcopyrite,  bornite, 
arsenopyrite,  tetrahedrite,  enargite,  stibnite,  cinnabar,  gold, 
argentite,  magnetite,  hematite,  cassiterite,  wolframite,  molyb- 
denite. There  are  of  course  many  rare  minerals  besides  these. 

Veins  and  replacement  deposits  have  been  grouped  by  Lind- 
gren  under  three  large  divisions  (a,  6,  c  below)  according  to  the 
temperature  of  formation,  which  in  general  depends  upon  the 
depth.  Certain  minerals  are  more  or  less  characteristic  of  each 
of  these  groups,  although  a-quartz,  calcite,  pyrite,  chalcopyrite, 
sphalerite,  galena,  and  gold  have  a  considerable  range  of  tem- 
perature and  hence  are  common  to  all  three  groups. 

(d)  High -Temperature  Deposits.     (Temperature  300°-500°C.  ± ). 

The  presence  of  such  minerals  as  tourmaline,  garnet,  topaz, 
lepidolite,  muscovite,  biotite,  apatite,  magnetite,  ilmenite,  and 
cassiterite  is  an  indication  that  the  deposit  was  formed  at  con- 
siderable depth  (since  exposed  by  erosion)  under  great  hydro- 


THE  ORIGIN  OF  MINERALS  457 

static  pressure  and  at  comparatively  high  temperatures.  Among 
high-temperature  deposits  may  be  classed:  (1)  cassiterite 
(or  tin-stone)  veins,  (2)  wolframite  veins,  (3)  molybdenite 
veins,  (4)  certain  gold-quartz  veins  (Juneau  region,  Alaska), 

(5)  gold    tellurid    veins    of   Western   Australia,  and    (6)  gold- 
tourmaline  veins  of  Meadow  Lake  district,  California. 

(6)  Intermediate-Temperature    Deposits    (Temperature   150°- 
300°C.±). 

These  are  distinguished  by  the  absence  of  most  of  the  minerals 
mentioned  under  (a)  and  instead  we  have  as  dominant  gangue 
minerals  quartz,  calcite,  dolomite,  siderite,  and  barite,  and  as  ore- 
minerals,  pyrite  (rarely  pyrrhotite),  chalcopyrite,  bornite,  galena, 
arsenopyrite,  tetrahedrite,  enargite,  sphalerite,  and  gold.  Hydro- 
thermal  alteration  product  minerals  are  usually  abundant  and 
include  sericite,  chlorite,  and  albite  (probably  a  low  temperature 
form).  Among  prominent  examples  of  intermediate  deposits 
are  the  following:  (1)  gold-quartz  veins  of  California,  (2)  silver- 
lead  veins  of  northern  Idaho,  (3)  silver-lead  replacements  of 
limestone  (Leadville,  Colo.,  Tintic,  Utah),  (4)  silver-cobalt  veins 
of  Ontario  (Canada),  (5)  copper  veins  of  Butte,  Montana,  and 
(6)  pyritic  replacements,  Shasta  County,  California. 

(c)  Low-Temperature  Deposits  (Temperature  50-150°C.±). 

Certain  deposits  believed  to  have  been  formed  comparatively 
near  the  surface  at  relatively  low  temperature  and  slight  hydro- 
static pressure  are  characterized  by  the  presence  of  such  minerals 
as  adularia,  amethyst,  quartz,  chalcedony,  opal,  fluorite,  alunite, 
and  in  a  few  cases  zeolites,  in  addition  to  ubiquitous  quartz  and 
calcite.  The  distinctively  high-temperature  minerals  are  absent. 
Of  the  ore  minerals,  pyrite,  marcasite,  cinnabar,  and  stibnite  are 
typical,  though  many  others  occur. 

Included  in  the  low  temperature  deposits  are  the  following: 
(1)  Tertiary  gold-quartz  veins  of  Nevada  and  Idaho,  (2)  gold 
tellurid  veins  of  Cripple  Creek,  Colorado,  (3)  silver-gold  ores  of 
Tonopah,  Nevada,  (4)  gold  deposits  of  Goldfield,  Nevada* 


458        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

(5)  mixed  ores  of  the  San  Juan  region,  Colorado,  (6)  cinnabar 
deposits,  and  (7)  stibnite  deposits. 

(d)  Stages  in  Mineral  Formation. 

While  occasionally  two  minerals  may  have  crystallized  out 
simultaneously  as  in  the  case  of  graphic  granite  (quartz  and 
orthoclase),  the  microscopic  study  of  ores  in  polished  surfaces 
proves  that  the  minerals  are  formed  in  stages  one  after  another. 
Most  of  the  copper  deposits  show  the  following  succession  of  ore- 
minerals:  (1)  pyrite,  (2)  chalcopyrite,  (3)  bornite,  (4)  chalcocite. 
This  succession  is  doubtless  due  to  gradual  decrease  of  temperature. 
In  the  high-temperature  veins  we  have  not  only  characteristic 
high-temperature  minerals  but  also  lower  temperature  minerals 
formed  at  later  stages.  It  is  thus  possible  in  many  cases  to 
determine  the  history  of  the  deposit. 

Several  more  or  less  distinct  periods  of  deposition  brought 
about  by  ascending  solutions  may  often  be  recognized,  but  still 
more  striking  are  the  results  brought  about  by  descending 
solutions. 

(e)  Ore-Deposits  not  Related  to  Igneous  Intrusions. 

The  three  clases  of  deposits  mentioned  above  are  believed  to 
have  had  a  genetic  connection  with  igneous  rocks.  In  other 
classes  of  ore-deposits  there  seems  to  be  no  possible  connection 
with  igneous  intrusions  and  so  they  are  believed  to  have  been 
formed  by  meteoric  waters.  As  many  of  these  minerals  are 
sulfids,  the  solutions  that  deposited  them  are  supposed  to  have 
been  alkaline;  hence  an  artesian  circulation  is  postulated. 

Good  examples  of  such  deposits  are  the  lead  and  zinc  ores  of 
the  Joplin  district  of  southwestern  Missouri  and  adjoining  portions 
of  Kansas  and  Oklahoma.  The  principal  gangue  minerals  are 
chalcedony  (very  little  quartz  is  present),  dolomite,  and  calcite, 
and  the  ore-minerals  are  sphalerite,  galena,  marcasite,  and  pyrite 
with  a  very  little  chalcopyrite.  The  silicates  of  hydrothermal 
deposits  are  entirely  absent  and  there  are  no  igneous  rocks  of 
any  kind  in  the  region. 


THE  ORIGIN  OF  MINERALS  459 

The  copper  ore  of  the  "Red-beds"  type  found  in  New  Mexico, 
Texas,  Oklahoma,  and  other  western  states  is  another  good 
example  of  this  class.  Nodules  and  wood  replacements  in 
sandstones  and  shales  contain  varying  amounts  of  chalcopyrite, 
bornite,  chalcocite,  and  covellite  in  addition  to  pyrite  and  hematite. 
Polished  surfaces  of  wood  replacements  show  cell-structure  under 
the  microscope. 

(/)  Zone  of  Oxidation. 

Most  of  the  vein  minerals  mentioned  under  (a),  (6),  (c)  above 
are  unstable  when  subject  to  weathering  processes  which  become 
active  when  the  deposits  are  exposed  by  erosion.  By  means  of 
meteoric  water  carrying  oxygen  the  sulfids  are  changed  first  to 
sulfates  and  then  to  oxids  or  hydroxids.  The  exposed  oxidized 
portion  of  a  vein  or  replacement  deposit  is  called  the  gossan 
or  "iron-hat."  At  the  surface  a  more  or  less  cellular  rusty 
outcrop  of  quartz  occurs.  This  is  the  leached  zone.  The  oxid- 
ized zone  usually  continues  for  some  distance  below  this  leached 
material  and  in  its  lower  portion  often  contains  oxids,  carbonates, 
or  sulfates. 

The  following  are  some  of  the  characteristic  minerals  of  the 
oxidized  zone  listed  according  to  metallic  content: 

Iron.  Limonite,  goethite,  hematite,  turyite,  copiapite  (Fe4- 
(OH)2(SO4)5-17H2O),  melanterite  (FeS04-7H2O),  and  jarosite. 

Copper.  Cuprite,  melaconite  (CuO(H20)J,  copper,  mala- 
chite, azurite,  chalcanthite,  brochantite,  atacamite  (Cu2(OH)3- 
Cl),  olivenite  (Cu2(OH)AsO4),  and  chrysocolla. 

Zinc.  Smithsonite,  calamine,  hydrozincite  (Zn3(OH)4CO3), 
and  goslarite  (ZnSO4-7H20). 

Lead.  Cerussite,  anglesite,  pyromorphite,  mimetite,  vanadi- 
nite,  and  wulfenite. 

Manganese.     Psilomelane,  pyrolusite,  and  manganite. 

Silver.     Silver  (also  original)  and  cerargyrite. 

Mercury.     Mercury  (Hg)  and  calomel  (Hg2Cl2). 

Cobalt.     Erthyrite  (Co3(As04)2-8H2O). 


460        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Nickel.     Annabergite  (Ni3(As04)2-8H2O). 
Antimony.     Stibiconite. 

Molybdenum.  Molybdite  (FeMo(V7H2O)  and  powellite  (Ca- 
Mo04). 

(g)  Supergene  Enrichment. 

Very  often,  especially  in  copper  mines,  there  is  found  below 
the  oxidized  zone  and  above  the  original  zone  of  sulfids  an 
enriched  zone  usually  called  the  zone  of  secondarj^  sulfid  enrich- 
ment. This  was  first  recognized  by  American  mining  geologists 
in  1900. 

In  the  case  of  copper  deposits  the  prominent  minerals  of 
this  zone  are  chalcocite  and  covellite.  These  have  been  formed 
by  the  action  of  soluble  sulfates  produced  by  the  oxidation  of 
sulfids,  principally  pyrite,  upon  the  unaltered  sulfids  below, 
such  as  pyrite  and  chalcopyrite.  Polished  surfaces  of  the  ores 
show  areas  or  spots  of  pyrite,  chalcopyrite,  or  bornite  bordered 
by  rims,  or  penetrated  by  veinlets,  of  chalcocite  or  covellite. 
Oxidation  products  such  as  limonite,  cuprite,  melaconite  (CuO- 
(H2O)X)  malachite,  brochantite,  or  chrysocolla  may  also  be 
present,  but  these  have  been  formed  at  a  later  period. 

Although  much  of  the  chalcocite  is  formed  by  meteoric  waters, 
some  investigators,  among  them  the  author,  believe  that  in  the  case 
of  certain  deposits  such  as  those  of  Butte,  Montana,  (where  the 
chalcocite  is  found  at  depths  of  over  3000  ft.)  much  of  the  chalco- 
cite is  formed  by  ascending  solutions.  This  now  seems  certain 
in  the  light  of  the  recent  discovery  (Schneiderhohn,  1920)  that 
some  chalcocite  was  evidently  formed  above  91°C.,  as  it  is  a 
paramorph  of  /3-Cu2S  (formed  below  91°C.)  after  a-Cu2S  (formed 
above  91°C.). 

It  is  usual  for  geologists  and  mining  engineers  to  call  minerals 
or  ores  formed  by  ascending  solutions  " primary"  and  those 
formed  by  descending  solutions  "secondary."  This,  however, 
is  not  the  original  use  of  these  terms.  A  primary  mineral  is  an 
original  mineral  and  a  secondary  mineral  is  one  that  is  formed 


THE  ORIGIN  OF  MINERALS  461 

subsequently.  The  ambiguity  in  the  use  of  these  words  may  be 
avoided  by  the  use  of  the  terms  hypogene  and  supergene  intro- 
duced by  Ransome.  A  hypogene  ore  or  mineral  is  one  formed  by 
ascending  solutions,  while  a  supergene  ore  or  mineral  is  one 
formed  by  descending  solutions.  The  terms  primary  and 
secondary  may  then  be  used  in  their  original,  mineralogical 
sense. 

Hypogene  and  supergene  chalcocite  may  usually  be  distin- 
guished by  a  careful  microscopic  examination  of  polished  surfaces. 


PART  IV 
THE  DETERMINATION  OF  MINERALS 

The  great  majority  of  mineral  specimens  may  be  recognized 
after  careful  inspection,  but  such  sight  recognition  requires 
considerable  experience  and  until  one  has  gained  this  experience 
by  handling  and  testing  many  minerals  he  must  use  physical  and 
chemical  tests  in  determining  the  mineral.  If  there  is  no  clue  to 
the  mineral,  it  is  better  to  use  determinative  tables  than  to  make 
tests  at  random.  Two  tables  have  been  prepared  for  this  purpose. 
Table  I  makes  use  of  physical  tests,  structure,  and  crystal  form, 
and  so  may  be  employed  in  the  field.  Table  II  makes  use  of 
chemical  and  blowpipe  tests  with  physical  tests  used  as  con- 
firmatory tests.  Its  use  is  of  course  limited  to  the  laboratory 
and  as  minerals  do  not  always  show  characteristic  crystal  form, 
structure,  color,  etc.,  it  is  of  more  general  application  than 
Table  I. 

It  should  be  emphasized  that  these  or  similar  tables  do  not 
determine  a  mineral,  but  simply  aid  one  in  the  determination. 
The  description  of  the  suspected  mineral  should  always  be  con- 
sulted and  additional  confirmatory  tests  made.  If  this  is  not 
done  the  use  of  the  tables  becomes  mechanical  and  no  progress  is 
made. 

The  very  common  minerals  are  given  in  capitals.  They 
should  be  considered  first,  for  most  of  the  minerals  found  are  the 
common  ones.  There  is  also  a  chance  that  the  mineral  specimen 
one  attempts  to  determine  is  not  included  in  the  list  of  175 
treated  in  this  book.  If  the  mineral  does  not  agree  with  any 
given  in  this  book,  then  it  may  be  well  to  consult  a  larger  book 
such  as  Brush  and  Penfield's  Determinative  Mineralogy  and 
Blowpipe  Analysis. 

463 


464        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Remarks  on  the  Use  of  Table  I.  In  Table  I  the  first  division 
is  based  upon  crystal  form.  Crystal  form  is  highly  characteristic 
and  many  minerals  may  be  distinguished  by  crystal  form  alone. 
A  hand  lens  (a  triple  aplanat  with  focus  of  about  18  mm.  is 
recommended)  will  be  found  very  useful  in  examining  small 
crystals.  The  simple  reflection  goniometer  described  on  p.  59 
may  be  used  to  advantage  in  determining  a  mineral  by  its  crystal 
form.  It  should  be  noted  that  isomorphous  minerals,  such 
as  barite  and  celestite,  have  nearly  identical  interfacial  angles, 
also  that  for  all  isometric  crystals  the  angles  between  corre- 
sponding faces  are  identical. 

On  account  of  its  constancy,  cleavage  is  one  of  the  best  means 
of  identifying  minerals.  Cleavage  should  be  recognized  by 
step-like  surfaces  rather  than  by  cracks  as  these  may  be  due  to 
other  causes. 

The  minerals  with  metallic  luster  are  quite  constant  in  color 
provided  a  fresh  fracture  is  used.  For  some  non-metallic  min- 
erals the  color  is  characteristic  and  practically  constant  but  for 
others  there  is  great  variation. 

The  specific  gravity  is  one  of  the  most  constant  properties  of 
minerals  and  for  pure  massive  specimens  without  cleavage  or 
structure  it  is  one  of  the  best  means  of  identification.  In  divi- 
sions B,  C,  and  D  of  Table  I  the  minerals  in  each  subdivision 
are  arranged  according  to  increasing  specific  gravity.  On 
p.  150  will  be  found  a  list  of  the  minerals  treated  in  this  book 
arranged  according  to  specific  gravity. 

Remarks  on  the  Use  of  Table  II 

In  table  II  the  important  tests  are  blowpipe  and  wet  tests  and 
so  the  homogeneity  and  purity  of  the  mineral  are  important. 
All  the  tests  must  of  course  be  made  of  the  same  kind  of  material. 
If  possible  the  impurity  should  be  removed,  if  not,  its  effect 
must  be  taken  into  account.  One  mineral  often  may  be  removed 
from  another  by  means  of  a  magnet,  by  panning,  by  specific 
gravity  separation  in  a  heavy  solution,  or  by  treatment  with  dilute 


THE  DETERMINATION  OF  MINERALS  465 

acid  in  case  one  of  them  is  insoluble.  The  homogeneity  cannot 
always  be  judged  by  inspection  even  with  the  hand  lens.  It  is 
often  necessary  to  examine  the  specimen  in  thin  sections  or  frag- 
ments to  see  whether  or  not  it  is  homogeneous. 

The  two  large  divisions  of  Table  II  are  (A)  non-metallic 
luster  and  (B)  metallic  luster,  but  if  the  luster  is  in  doubt  it 
may  be  disregarded,  in  which  case  the  sulfids  of  division  7  must 
be  considered  with  division  8  and  the  remainder  of  division  7 
with  division  9.  The  minerals  are  arranged  as  far  as  possible 
according  to  the  acid  radical.  This  is  the  arrangement  of  minerals 
used  in  Part  II  (The  Description  of  Important  Minerals). 

The  Determination  of  Minerals  by  Optical  Tests 

The  determination  of  minerals  by  optical  tests  is  practically 
limited  to  those  that  are  fairly  transparent  in  thin  slices  or  frag- 
ments, but  many  apparently  opaque  minerals  become  transparent 
or  translucent  when  reduced  to  fragments.  For  many  non- 
metallic  minerals  the  optical  determinations  are  easier  to  make 
and  are  more  satisfactory  than  blowpipe  and  chemical  tests.  It 
should  also  be  emphasized  that  some  distinctions  which  are 
practically  impossible  by  chemical  methods  may  be  made  by 
means  of  the  polarizing  microscope.  For  example,  the  distinc- 
tions between  orthoclase  and  microcline,  and  quartz  and  chal- 
cedony. A  convenient  method  for  optical  determination  of 
minerals  is  to  reduce  the  mineral  to  a  coarse  powder  by  pounding 
rather  than  grinding.  The  coarsest  fragments  that  go  through  a 
100-mesh  sieve  are  examined  in  some  liquid  such  as  clove  oil  or 
cinnamon  oil,  and  the  shape,  color,  pleochroism,  relief,  inter- 
ference colors,  extinction  angle,  sign  of  elongation,  etc.  are  noted. 
The  indices  of  refraction  with  reference  to  a  standard  set  of 
liquids  are  determined.  A  list  of  minerals  arranged  according 
to  indices  of  refraction  will  be  found  on  p.  207.  The  determi- 
nation of  the  indices  of  refraction  is  often  sufficient  to  completely 
determine  a  mineral  but  it  is  advisable  to  make  confirmatory 
chemical  tests.  Some  of  these  may  be  performed  on  a  small  scale 


466        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

on   a  glass  slip   and  the  precipitates  which  are  characteristic 
may  be  examined  with  the  polarizing  microscope. 

SYNOPSIS  OF  TABLE  I 

A.  The  mineral  appears  in  distinct  euhedral  crystals. 

I.  The  crystal  habit  is  equidimensional  (isometric  or  pseudo-iso- 
metric). 
II.  The  crystal  habit  is  tabular. 

III.  The  crystal  habit  is  pyramidal. 

IV.  The  crystal  habit  is  prismatic. 

V.  The  crystal  habit  is  complex  and  not  previously  included. 
VI.  The  crystals  are  obviously  twins. 

B.  The  mineral  has  well-defined  structure. 

I.  The  structure  is  columnar  or  bladed. 
II.  The  structure  is  fibrous. 

III.  The  structure  is  foliated  or  micaceous. 

IV.  The  structure  is  colloform. 

V.  The  structure  is  oolitic  or  pisolitic. 

C.  The  mineral  has  good  to  perfect  cleavage  (or  parting). 

I.  Cleavage  in  one  direction. 
II.  Cleavage  in  two  directions  at  right  angles  or  nearly  so. 

III.  Cleavage  in  two  directions  at  oblique  angles. 

IV.  Cleavage  in  three  directions  at  right  angles. 

V.  Cleavage  in  three  directions;  two  at  right  angles,  the  third  at  an 

oblique  angle. 

VI.  Cleavage  in  three  directions  at  oblique  angles. 
VII.  Cleavage  in  four  or  more  directions. 

D.  The   mineral  in  massive  without  prominent  cleavage  or  any  particular 

structure. 
I.  Luster,  metallic  or  submetallic. 

1.  Color — black  to  dark  gray. 

2.  Color — tin-white  to  light  gray. 

3.  Color — brass,  bronze,  red,  brown,  etc. 
II.  Luster,  adamantine  or  subadamantine. 

III.  Luster,  non-metallic  (and  not  adamantine) : 

1.  Color — red  or  pink. 

2.  Color — yellow. 

3.  Color — green. 


THE  DETERMINATION  OF  MINERALS  467 

4.  Color— blue. 

5.  White,  colorless,  or  nearly  so. 

6.  Color — gray. 

7.  Color— brown. 

8.  Black. 

A.    THE  MINERAL  APPEARS  IN  DISTINCT  EUHEDRAL 
CRYSTALS. 

I.  THE  CRYSTAL  HABIT  IS  EQUIDIMENSIONAL  (ISOMETRIC 
OR  PSEUDO-ISOMETRIC). 

1.  Cubes  or  cube-like  forms. 

ANHYDRITE,  p.  330.     Pseudo-cubes. 
Apophyllite,  p.  406.     Cleavage  in  one  direction. 
Argentite,  p.  228.     Metallic  luster. 
CALC1TE,  p.  291.     Rhombohedral  cleavage. 
Cerargyrite,  p.  253.     Adamantine  luster. 
Chabazite,  p.  410.     Rhombohedrons  of  85°  angle. 
Cuprite,  p.  267.     Adamantine  luster. 
Cryolite,  p.  256.     Pseudo-cubic. 
FLUORITE,  p.  254.     Octahedral  cleavage. 
GALENA,  p.  228.     Metallic  luster.     Cubic  cleavage. 
HALITE,  p.  251.     Cubic  cleavage. 
Jarosite,  p.  336.     Pseudo-cubic  rhombohedrons. 
PYR1TE,  p.  236.     Metallic  luster.     Crystals  usually  stri- 
ated. 

QUARTZ,  p.  258.     Rhombohedron  of  86°  angle. 
Smaltite,  p.  238.     Metallic  luster. 
Sylvite,  p.  253.     Cubic  cleavage;  resembles  halite. 

2.  Octahedrons  or  pseudo-octahedrons. 

CHROMITE,  p.  281.     Metallic  luster. 
Cuprite,  p.  267.     Adamantine  luster. 
Diamond,  p.  213.     Adamantine  luster. 
FLUORITE,  p.  254.     Octahedral  cleavage. 
Franklinite,  p.  280.     Metallic  luster. 
GALENA,  p.  228.     Metallic  luster.     Cubic  cleavage. 
MAGNETITE,  p.  279.     Metallic  luster. 
PYRITE,  p.  236.     Metallic  luster. 
SPHALERITE,  p.  231.     Dododecahedral  cleavage. 
Spinel,  p.  278.     No  cleavage. 


468        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

3.  Tetrahedrons  or  pseudo -tetrahedrons. 

CHALCOPYRITE,  p.  244.     Metallic  luster. 
SPHALERITE,  p.  231.   Adamantine  luster.    Good  cleavage. 
TETRAHEDRITE,  p.  247.     Metallic  luster. 

4.  Dodecahedrons. 

Cuprite,  p.  267.     Adamantine  luster. 
Diamond,  p.  213.     Adamantine  luster. 
GARNET,  p.  371.     Usually  red  or  brown 
MAGNETITE,  p.  279.     Metallic  luster 

6.  Trapezohedrons. 

Anal  cite,  p.  410.     Usually  colorless. 
GARNET,  p.  371.     Usually  red  or  brown 
Leucite,  p.  355.     Usually  white. 

6.  Pyritohedrons. 

PYRITE,  p.  236.     Metallic  luster. 

II.  THE  CRYSTAL  HABIT  IS  TABULAR. 

1.  Rhombic  cross -section. 

Anglesite,  p.  329.     Adamantine  luster. 
ARSENOPYRITE,  p.  240.     Metallic  luster. 
BARITE,  p.  327.     Vitreous  luster. 
Celestite,  p.  328.     Vitreous  luster. 
GYPSUM,  p.  332.     Monoclinic. 
Marcasite,  p.  239.     Metallic  luster. 
MUSCOVITE,  p.  393.     Perfect  cleavage. 

2.  Square  cross-section. 

Apophyllite,  p.  406.     Cleavage  in  one  direction. 
Wulfenite,  p.  340.     Adamantine  luster. 

3.  Rectangular  cross-section. 

BARITE,  p.  327.     Good  cleavage. 

4.  Hexagonal  cross-section. 

APATITE,  p.  313.     Practically  no  cleavage. 
CALCITE,  p.  291.     Good  cleavage. 
CHLORITE,  p.  397.     Perfect  basal  cleavage. 
CORUNDUM,  p.  268.     Triangular  markings  on  base. 
Covellite,  p.  234.     Metallic  luster. 
Dahllite,  p.  315.     Minute  crystals. 
GRAPHITE,  p.  216.     Metallic  luster. 
HEMATITE,  p.  270.     Metallic  luster. 


THE  DETERMINATION  OF  MINERALS  469 

Ilmenite,  p.  310.     Metallic  luster. 

MICA  GROUP,  p.  392.     Perfect  cleavage  in  one  direction. 

Molybdenite,  p.  227.     Metallic  luster. 

Polybasite,  p.  249.     Metallic  luster. 

Stephanite,  p.  248.     Metallic  luster. 

HI.  THE  CRYSTAL  HABIT  IS  PYRAMIDAL. 

1.  Rhombic  cross -section. 

Anglesite,  p.  329.     Adamantine  luster. 
SULFUR,  p.  217.     Adamantine  luster. 

2.  Square  cross -section. 

Apophyllite,  p.  406.     Cleavage  in  one  direction. 
CASSITERITE,  p.  272.     Adamantine  luster. 
Scheelite,  p.  339.     Sub-adamantine  luster. 
Vesuvianite,  p.  378.     Low  pyramidal  habit. 
Zircon,  p.  379.     Adamantine  luster. 

3.  Hexagonal  cross-section. 

CALCITE,  p.  291.     Rhombohedral  cleavage. 
CERUSSITE,  p.  305.     Adamantine  luster. 
CORUNDUM,  p.  268.     Steep  hexagonal  bipyramids. 
DOLOMITE,  p.  296.     Resembles  calcite. 
HEMATITE,  p.  270.     Metallic  luster. 
Nitratine,  p.   322.     Resembles  cleavage    rhombohedron 

of  calcite. 

Pyrargyrite,  p.  246.     Metallic-adamantine  luster. 
QUARTZ,  p.  258. 

Rhodochrosite,  p.  300.     Resembles  calcite. 
SIDERITE  p.  299.     Resembles  calcite. 
Witherite,  p.  305.     Resembles  quartz  in  form. 

IV.  THE  CRYSTAL  HABIT  IS  PRISMATIC. 
1.  Rhombic  cross-section. 

Adularia,  p.  346.     Cleavage  oblique  to  prism. 
Anglesite,  p.  329.     Adamantine  luster. 
Anthophyllite,   p.   364.     Resembles   tremolite   and   horn- 
blende. 

BARITE,  p.  327.     Prismatic  and  basal  cleavage. 
HORNBLENDE,  p.  365.     Prism  angle  56°  and  124°. 
Sillimanite,  p.  383.     One  perfect  cleavage. 
Staurolite,  p.  387.     No  cleavage. 


470        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

Topaz,  p.  380.     Good  cleavage  normal  to  prism  faces. 
Tremolite,  p.  364.     Prism  angle  56°  and  124°'. 

2.  Square  cross-section. 

Andalusite,  p.  381.     No  cleavage. 
Apophyllite,  p.  406.     Good  cleavage  in  one  direction. 
Augite,  p.  361. 

Diopside,  p.  360.    Imperfect  cleavage,  often  parting  1101. 
Natrolite,  p.  411.     Prisms  terminated  by  low  pyramid. 
ORTHOCLASE,  p.  344.     Monoclinic. 
PYROXENE,  p.  359.     Monoclinic. 
Rutile,  p.  273.     Metallic-adamantine  luster. 
Scapolite,  p.  377.     Prisms  with  low  pyramids. 
Topaz,  p.  380.     Good  cleavage  normal  to  prism  faces. 
Vesuvianite,  p.  378.     Prisms  with  low  pyramids. 
Zircon,    p.    379.     Prisms    with    low    pyramids.     Small 
crystals. 

3.  Hexagonal  cross-section. 

APATITE,  p.  313.     Practically  no  cleavage. 

Aragonite,    p.     302.     Resembles     calcite     but     cleavage 

imperfect. 

Beryl,  p.  368.     No  cleavage. 

CALCITE,  p.  291.     Perfect  rhombohedral  cleavage. 
CORUNDUM,  p.  268.     Triangular  markings  on  base. 
Dahllite,  p.  315.     Minute  crystals. 
EPIDOTE,  p.  384.     Good  cleavage  in  one  direction. 
HORNBLENDE,  p.  365.     Prismatic  cleavage. 
Mimetite,  p.  316.     Adamantine  luster. 
Nepheline,  p.  355.     Crystals  small. 
Pyrargyrite,  p.  246.     Metallic-adamantine  luster. 
Pyromorphite,  p.  315.     Adamantine  luster. 
QUARTZ,  p.  258.     No  cleavage. 

TOURMALINE,  p.  388.     No  cleavage.     3P  intersect  in  A3. 
Tremolite,    p.    364.     Resembles    hornblende    in    form. 
Vanadinite,  p.  317.     Adamantine  luster. 
Willemite,  p.  375.     Hexagonal  prism  1120  and  rhombohe- 

dron  1011. 

V.  CRYSTAL     HABIT    COMPLEX    AND     NOT    PREVIOUSLY 
INCLUDED. 

Axinite,  p.  391.     Triclinic. 

CALCITE,  p.  291.     Perfect  rhombohedral  cleavage. 


THE  DETERMINATION  OF  MINERALS  471 

Colemanite,  p.  323.     Perfect  cleavage. 
Datolite,  p.  391.     No  cleavage;  monoclinic. 
Diopside,  p.  360.     Imperfect  cleavage,  often  parting. 
Microcline,  p.  347.     Apparently    monoclinic;     resembles 

orthoclase. 
ORTHOCLASE,  p.  344.     Monoclinic. 

VI.  THE  CRYSTALS  ARE  OBVIOUSLY  TWINS. 
1.  Contact  twins. 

Albite,  p.  350.     Usually  Carlsbad  twin. 
Aragonite,  p.  302.     Twin  plane  =  JHO). 
CALCITE,  p.  291.    See  Figs.  458-461. 
CASSITER1TE,  p.  272.     Resembles  rutile. 
CERUSSITE,  p.  305.     Crystals  at  60°  angles. 
Diamond,  p.  213.     Spinel  twin  like  Fig.  438,  p.  278. 
EPIDOTE,  p.  384.     See  Fig.  557. 
GYPSUM,  p.  332.     Twin  plane  usually  { 100 } . 
HORNBLENDE,  p.  365.     Twin  plane  =  JIOOJ. 
Microcline,  p.  347.     Resembles  orthoclase. 
ORTHOCLASE,  p.  344.     See  Figs.  508  and  509. 
PYROXENE,  p.  359.     Twin  plane  =  { 100  j . 
Rutile,  p.  273.    See  Figs.  433-436. 
Spinel,  p.  278.     Octahedrons  twinned  on  {ill}. 

2.  Penetration  twins. 

CALCITE,  p.  291. 

CERUSSITE,  p.  305.     Crystals  at  60°  angles. 
FLUORITE,  p.  254.     Twin  axis    =  cube  diagonal. 
Microcline,  p.  347.     Twin  axis  =  c-axis. 
ORTHOCLASE,  p.  344.     Twin  axis  =  c-axis. 
PYRITE,  p.  236.     See  Fig.  257,  p.  127. 

3.  Polysynthetic  twinning  striations. 

CALCITE,  p.  291.     Striations  parallel  to  long  diagonal. 
CORUNDUM,  p.  268.     Rhombohedral  parting. 
GALENA,  p.  228.    See  Fig.  255  p.  127. 
PLAGIOCLASE,    p.    348.    Striations    usually    on    best 

cleavage. 

Rutile,  p.    273.     See  Fig.  434. 
SPHALERITE,  p.  231.     Twin  plane  =  {ill}. 


472        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


B.  THE  MINERAL  HAS  WELL-DEFINED  STRUCTURE. 
I.  THE  STRUCTURE  IS  COLUMNAR  OR  BLADED. 
1.  The  mineral  is  scratched  by  the  finger-nail. 


Sp.  gr. 


Remarks 


STIBNITE,  p.  225  

4.5 

Good   cleavage   in  one 

Bismuthinite,  p.  226  

6.4 

direction. 
Resembles  stibnite. 

2.  The   mineral   is   not  scratched  by   the  finger-nail,   but   is 
scratched  by  a  knife  blade. 


Colemanite,  p.  323  
CALCITE,  p.  291  

Wollastonite,  p.  369  
Aragonite,  p.  302  

2.4 

2.7 

2.8 
2.9 

Perfect  cleavage  in  one 
direction. 
Perfect     cleavage     ob- 
lique to  length. 
Good  cleavage. 
Imperfect     cleavage 

CALAMINE,  p.  376. 

3  4 

parallel  to  length. 
Subradiating  crusts. 

Kyanite,  p  382 

3  6 

Perfect  cleavage. 

Strontianite,  p.  304  

3.7 

Resembles  aragonite, 

Goethite,  p  284 

4  3 

except  in  spec.  grav. 
Yellow-brown  streak. 

Manganite,  p.  285  
Enargite,  p.  250  

4.3 
4.4 

Dark-brown    streak. 
Metallic  luster. 
Grayish-black  streak. 
Metallic  luster. 

3.  The  mineral  is  not  scratched  by  a  knife  blade. 


Beryl,  p.  368  

2.7 

No  cleavage. 

Tremolite,  p.  364  

3.0 

Cleavage  in  two  direc- 

TOURMALINE, p.  388... 
HORNBLENDE,  p.  365.. 

Clinozoisite,  p.  386  
EPIDOTE,  p.  384 

3.1 
3.2 

3.3 
3  4 

tions  at  oblique  angles. 
No  cleavage. 
Dark  green  or  brown  to 
black;  perfect  cleavage. 
Gray  to  pale  green. 
Pistache  green  color. 

Vesuvianite,  p.  378  
Kyanite,  p.  382  

3.4 
3.6 

No  distinct  cleavage. 
Good    cleavage  in    one 

Wolframite,  p.  338  

7.2 

direction. 
Good  cleavage. 

THE  DETERMINATION  OF  MINERALS 


473 


II.  THE  STRUCTURE  IS  FIBROUS. 

1.  The  mineral  is  scratched  by  the  finger-nail. 


Sp.  gr. 

Remarks 

Ulexite,  p.  324  
Chrysotile,  p.  400  

1.6 
2.2 

Loosely    compacted 
"cotton-balls." 
Usually  occurs  in   ser- 

GYPSUM, p.  332     .  . 

2  3 

pentine. 
In  sedimentary  rocks. 

Brucite,  p  288 

2  4 

May  occur  in  serpentine. 

Pyrophyllite,  p.  401  
Pyrolusite,  p.  274  

2.8 
4.8 

Usually    radiating. 
Pearly  luster. 
Black.    Metallic  to  dull 

2.  The   mineral   is   not   scratched   by   the  finger-nail,  but  is 
scratched  by  a  knife  blade. 


Stilbite,  p.  409  
Natrolite,  p.  411  

ANTIGORITE,p.398..  .  . 
CALCITE,  p.  291  

TALC,  p.  400  

2.1 
2.2 

2.5 

2.7 

2.7 

Secondary    mineral    in 
cavities  or  seams. 
Secondarjr    mineral   in 
cavities  or  seams. 
In  serpentine. 
Perfect     cleavage     ob- 
lique to  length. 
Soapv  feel. 

Wollastonite,  p.  369  
Aragonite,  p.  302 

2.8 
2  9 

In  limestone. 
Imperfect  cleavage  par- 

Sillimanite, p.  383  

3.2 

allel  to  length. 
In  metamorphic  rocks. 

Strontianite,  p.  304  
Celestite,  p.  328 

3.7 
3  9 

Resembles  aragonite. 
Often  pale  blue. 

MALACHITE,  p.  307  
Goethite,  p.  284  
Manganite,  p.  285  

3.9 
4.3 
4.3 

Emerald  green. 
Yellow-brown  streak. 
Dark  -brown  streak. 

474         INTRODUCTION  TO  THE  STUDY  OF  MINERALS 
3.  The  mineral  is  not  scratched  by  a  knife  blade. 


Sp.  gr. 

Remarks   . 

Tremolite,  p.  364       .    . 

3.0 

Good   cleavage  ]   white, 

pale  green,  or  gray. 

Anthophyllite,  p.  364  

3.1 

Good    cleavage.     Usu- 

ally brown. 

Glaucophane,  p.  367  

3.1 

Good  cleavage.     Blue- 

black. 

TOURMALINE,  p.  388... 

3.1 

No  cleavage.     Usually 

black. 

HORNBLENDE,  p.  365.. 

3.2 

Good    cleavage.     Dark 

black. 


HI.  THE  STRUCTURE  IS  FOLIATED  OR  MICACEOUS. 
1.  The  mineral  is  scratched  by  the  finger-nail. 


GRAPHITE,  p.  216  
GYPSUM,  p.  332  

2.1 
2  3 

Black.     Metallic  luster. 
Gray  streak. 
Perfect  cleavage. 

Brucite,  p.  288  

2  4 

Perfect  cleavage. 

TALC,  p.  400  
CHLORITE,  p.  397  

Molybdenite,  p.  227  

2.7 

2.8 

4.7 

Soapy  feel. 
Light    to    dark    green. 
Flexible  plates. 
Green  streak  on  glazed 
paper. 

2.  The    mineral   is  not  scratched   by   the    finger-nail,   but  is 
scratched  by  a  knife  blade. 


Lepidolite,  p.  395 

2  8 

Perfect  cleavage     Lilac 

MUSCOVITE,  p.  393  
Phlogopite,  p.  397  

20 
.0 

2  8 

colored. 
Perfect  cleavage.  Light 
colored. 
Perfect    cleavage. 

BIOT1TE,  p    396  ...      . 

2  9 

Brown.    In  limestone. 
Perfect    cleavage. 

BARITE,  p.  328  

4.5 

Black  to  deep  brown. 
Very  heavy. 

THE  DETERMINATION  OF  MINERALS  475 

3.  The  mineral  is  not  scratched  by  a  knife  blade. 


Sp.  gr. 


Remarks 


Albite,  p.  350  
HEMATITE,  p.  270  

2.6 
5.2 

Twinning    lamellae    on 
ends  of  plates. 
Metallic   luster.       Red 
streak. 

IV.  THE  STRUCTURE  IS  COLLOFORM. 

1.  Mamillary,  botryoidal,  etc. 

2.  The   mineral   is   not   scratched   by   the   finger-nail,  but  is 

scratched  by  a  knife  blade. 


OPAL,  p.  262  
Gibbsite,  p.  286  

2.1 
2.4 

Usually  clear  and  color- 
less. 
Usually  in  bauxite. 

CALCITE,  p.  291  
Dahllite,  p.  315  

2.7 
3.0 

In  limestone. 
In  phosphorite. 

LIMONITE,  286  

MALACHITE,  p.  307  
Goethite,  p.  284 

3.8 

3.9 
4  3 

Yellow-brown     streak. 
Amorphous. 
Emerald  green. 
Yellow-brown      streak. 

SMITHSONITE,  p.  301.. 

4.4 

Crystalline. 
Curved  cleavage. 

3.  The  mineral  is  not  scratched  by  a  knife  blade. 


OPAL,  p.  262  
CHALCEDONY,  p.  261.  . 

PSILOMELANE,  p.  289.. 
Turyite,  p.  271  

2.1 

2.6 

4.2 
4.5 

Usually  clear  and  color- 
less. 
Waxy  to  dull.     Trans- 
lucent. 
Black.     Dull  luster. 
Red   streak.      Amor- 

HEMATITE, p.  270  
CASSITERITE,  p.  272.  .. 

5.2 
7.0 

phous. 
Red   streak.     Crystal- 
line. 
Colorless  streak.     Ada- 
mantine luster. 

476        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


V.  THE  MINERAL  HAS  AN  OOLITIC  OR  PISOLITIC  STRUC- 
TURE. 
1.  The  mineral  is  scratched  by  the  finger-nail. 


Sq.  gr. 


Remarks 


Glauconite,  p.  406 

2  3 

Green.     In  sandstones. 

CLIACHITE,  p.  287 

2  5 

Pale  colors  to  red.     In 

bauxite. 

2.  The    mineral   is   not   scratched   by   the  finger-nail,   but  is 
scratched  by  a  knife  blade. 


CLIACHITE,  p.  287  
CALCITE,  p.  291  

2.5 

2.7 

Pale  colors  to  red.      In 
bauxite. 
White   to  g  r  a  y  .      In 

COLLOPHANE,  p.  319.. 

SIDERITE,  p.  286  
LIMONITE,  p.  286  
HEMATITE,  p.  270  

2.8 

3.8 
3.8 
5.2 

limestones. 
Any  color.     In  phos- 
phorites. 
Brown  with  pale  streak. 
Yellow-brown  streak. 
Red  streak. 

3.  The  mineral  is  not  scratched  by  a  knife  blade. 


CHALCEDONY,  p.  261.  .. 

2.6 

Light  gray.     Quartz 
usually  present. 

C.  THE  MINERAL  DOES  NOT  APPEAR  IN  DISTINCT  CRYSTALS 
AND  HAS  NO  PARTICULAR  STRUCTURE,  BUT  DOES 
HAVE  GOOD  TO  PERFECT  CLEAVAGE  (OR  PARTING.) 
I.  CLEAVAGE  (OR  PARTING)  IN  ONE  DIRECTION. 
1.  The  mineral  is  scratched  by  the  finger-nail. 


GRAPHITE,  p.  216 

2  1 

Black.     Metallic  luster. 

GYPSUM,  p.  332  
Brucite,  p.  288  

2.3 

2.4 

Two  other  less  perfect 
cleavages. 
No    secondary     cleav- 

Vivianite,p. 318  
TALC,  p.  400 

2.6 
2.7 

ages. 
Blue. 
Soapy  feel. 

CHLORITE,  p.  397  
STIBNITE,  p.  225  
Molybdenite,  p.  227  
Bismuthinite,  p.  226  

2.8 
4.5 
4.7 
6.4 

Light  to  dark  green  . 
Plates  are  flexible. 
Cleavage  parallel  to 
length. 
Greenish    streak    on 
glazed  paper. 
Resembles  stibnite. 

THE  DETERMINATION  OF  MINERALS 


477 


2.  The   mineral   is   not   scratched   by   the  finger-nail,  but  is 
scratched  by  a  knife  blade. 


Heulandite,  p.  408  
Apophyllite,  p.  406  
Colemanite,  p.  323  :  

2.2 
2.3 

2.4 

Pearly  luster  on  cleav- 
age face.     Monoclinic. 
Pearly  luster  on  cleav- 
age face.     Tetragonal. 
Vitreous  luster. 

Lepidolite,  p.  395 

2.8 

Lilac  colored. 

MUSCOVITE,  p.  393  
Sericite,  p.  304  
Phlogopite,  p.  397  .  .  . 

2.8 
2.8 
2  8 

Light  colored. 
Scaly.  Fine-grained. 
Brown.     In  limestones. 

BIOTITE,  p.  396  

2.8 

Black  to  dark  brown. 

Kyanite,  p.  382  
Celestite,  p.  328  
Goethite,  p.  284  

3.6 
3.9 
4.3 

Bladed  structure. 
Resembles  barite. 
Cleavage  parallel  to 

Wolframite,  p.  338  

7.4 

length.    Yellow-brown 
streak. 
Black  submetallic  lus- 
ter. 

3.  The  mineral  is  not  scratched  by  a  knife  blade. 


Spodumene,  p.  370  
Diopside,  p.  360 

3.1 
3  2 

Also    prismatic    cleav- 
ages. 
Basal  parting.     Imper- 

EPIDOTE, p.  384 

3  4 

fect    prismatic    cleav- 
age. 
Pistache  green. 

Topaz,  p.  380  

3.5 

Cleavage  perpendicular 

Kyanite,  p.  382  

3.6 

to  prism  faces. 
Bladed  structure. 

CORUNDUM,  p.  268  
Goethite,  p.  284  

4.0 
4.3 

Basal  parting  with  tri- 
angular striations. 
Yellow-brown  streak. 

Wolframite,  p.  338  

7.4 

Black.     Submetallic 
luster. 

478         INTRODUCTION  TO  THE  STUDY  OF  MINERALS 

II.    CLEAVAGE    (OR    PARTING)    IN    TWO    DIRECTIONS    AT 
RIGHT  ANGLES  OR  NEARLY  SO. 

1. 


3.  The  mineral  is  not  scratched  by  a  knife  blade. 


Sp.  gr. 

Remarks 

Adularia,  p.  346  .  .  . 

2  6 

Usually  clear  and  color- 

ORTHOCLASE, p.  344.  .  . 
Microcline,  p.  347  

2.6 
2  6 

less.     Rhombic  habit. 
No  twinning  striations. 
Resembles     orthoclase. 

PLAGIOCLASE,  p.  348.. 
Scapolite,  p.  377 

2.6 

2  7 

May   show   cross- 
hatching. 
Usually  shows  twinning 
striations. 
Imperfect  cleavage 

Wollastonite,  p.  369  
Spodumene,  p.  370  

Diopside,  p.  360  
Augite,  p.  361  

2.8 
3.1 

3.2 
3.3 

Usually  in  limestones. 
Often  shows  pinacoidal 
parting. 
White  to  green. 
Dark  green  to  black. 

Rhodonite,  p.  362  
Rutile,  p.  273 

3.6 
4  2 

Usually  red.     Non-me- 
tallic. 
Dark  redj  metallic-ada- 

mantine luster. 

III.  CLEAVAGE    (OR     PARTING)    IN    TWO    DIRECTIONS    AT 
OBLIQUE  ANGLES. 

1. 

2.  The   mineral   is  not   scratched   by  the   finger-nail,  but  is 
scratched  by  a  knife  blade. 


Enargite,  p.  250 


I  4.4     |  Columnar  structure. 


3.  The  mineral  is  not  scratched  by  a  knife  blade. 


Tremolite,  p.  364  
Anthophyllite,  p.  364  
HORNBLENDE,  p.  365.. 

Titanite,  p.  412  

3.0 
3.1 
3.2 

3.5 

White,  gray,  pale  green. 
Usually  brown. 
Dark  green  or  brown  to 
black. 
Yellow  to  brown. 

THE  DETERMINATION  OF  MINERALS 


479 


IV,  CLEAVAGE  (OR    PARTING)    IN  THREE    DIRECTIONS  AT 
RIGHT  ANGLES. 

1.  The  mineral  is  scratched  by  the  finger-nail. 


Sp.  gr. 


Remarks 


Sylvite,  p.  253 2.0       Bitter  taste. 


2.  The   mineral   is   not   scratched   by   the   finger-nail,  but  is 
scratched  by  a  knife  blade. 


HALITE,  p.  251 

2  1 

Soluble  in  water  (taste). 

ANHYDRITE,  p.  330  
Cryolite,  p.  256     

2.9 
3.0 

Pseudo-cubic  cleavage. 
Almost    disappears    in 

GALENA,  p.  228  

7.5 

water. 
Lead     gray.      Metallic 
luster. 

3.  The  mineral  is  not  scratched  by  a  knife  blade. 


CORUNDUM,  p.  268  

4.0 

Very   hard.     Pseudo- 
cubic  parting. 

V.  CLEAVAGE    IN    THREE    DIRECTIONS;    TWO    AT    RIGHT 
ANGLES,  THE  THIRD  AT  AN  OBLIQUE  ANGLE. 

1.  The  mineral  is  scratched  by  the  finger-nail. 


GYPSUM,  p.  332.. 


2.3     I  Pearly  luster. 


2.  The   mineral   is   not   scratched   by   the  finger-nail,  but  is 
scratched  by  a  knife  blade. 


Celestite,  p.  328 

3  9 

Prismatic  cleavage  im- 

BARITE, p.  327  

4.5 

perfect. 
Prismatic  cleavage  per- 

fect. 

3.  The  mineral  is  not  scratched  by  a  knife  blade. 


Spodumene,  p.  370 3.1       Pinacoidal  parting. 


480        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


VI.  CLEAVAGE  IN  THREE  DIRECTIONS  AT  OBLIQUE  ANGLES 
(RHOMBOHEDRAL  CLEAVAGE). 

1. 

2.  The   mineral   is   not   scratched   by   the   finger-nail,   but  is 
scratched  by  a  knife  blade. 


Sp.  gr. 

Remarks 

CALCITE,  p.  291 

2  7 

Any  color. 

DOLOMITE,  p.  296  
Magnesite,  p.  298 

2.8 
3  1 

Resembles  calcite,  but 
cleavage  often  curved. 
Color  varies.    Chemical 

Rhodochrosite,  p.  300  
SIDERITE,  p.  299  

3.5 

3.8 

tests  necessary. 
Pink  to  red. 
Brown. 

VII.  CLEAVAGE   (OR  PARTING)   IN  FOUR  OR  MORE  DIREC- 
TIONS. 


2.  The   mineral   is   not  scratched  by  the   finger-nail,  but  is 
scratched  by  a  knife  blade. 


CALCITE,  p.  291 

2.7 

Rhombohedral  cleav- 

FLUORITE, p  254. 

3  2 

age  plus  parting. 
Octahedral  cleavage. 

SPHALERITE,  p.  231.... 

4.0 

Dodecahedral  cleavage. 
Adamantine  luster. 

3.  The  mineral  is  not  scratched  by  a  knife  blade. 


Scapolite,  p.  377  
Diamond,  p.  213  
CORUNDUM,  p.  268  

MAGNETITE,  p.  279.... 

2.7 
3.5 
4.0 

5.1 

Prismatic  cleavage. 
Adamantine  luster. 
Rhombohedral  and 
basal  parting. 
Octahedral  parting. 
Metallic  luster. 

THE  DETERMINATION  OF  MINERALS 


481 


D.  THE  MINERAL  IS  MASSIVE  WITHOUT  PROMINENT   CLEAV- 
AGE OR  ANY  PARTICULAR  STRUCTURE. 

I.  LUSTER  METALLIC   OR  SUBMETALLIC.     (The  thin  edges 
are  opaque.) 


1.  Color  black  to  dark  gray. 


Sp.  gr. 

Remarks 

(a)    The    mineral  is 

GRAPHITE,  p.  216... 

2.1 

Streak    gray     on 

scratched     by    the 

glazed  paper. 

finger-nail. 

STIBNITE,  p.  215  

4.5 

Lead-gray  streak. 

Molybdenite,  p.  227  .  .  . 

4.7 

Greenish    streak    on 

glazed  paper. 

Pyrolusite,  p.  274  

4.8 

Streak  black. 

Argentite,  p.  228  

7.3 

Very  sectile. 

(6)   The    mineral    is 

TETRAHEDRITE,    p. 

not   scratched   by 

247  

4.7 

Very  brittle. 

the  finger-nail,  but 

CHALCOCITE,  p.  230. 

5.7 

Subsectile. 

is   scratched    by   a 

Pyrargyrite,  p.  246  

5.8 

Red  on  thin  edges. 

knife  blade. 

Stephanite,  p.  248  

6.2 

Black  on  thin  edges. 

Wolframite,  p.  338  

7.4 

Submetallic  luster. 

(c)  The    mineral    is 

PSILOMELANE,  p. 

4.2 

No  cleavage  or  struc- 

not scratched  by  a 

273. 

ture. 

knife  blade. 

Rutile,  p.  273  

4.2 

Reddish  brown. 

CHROMITE,  p.  281... 

4.4 

Brown    streak.     Oc- 

curs in  serpentine. 

Ihnenite,  p.  310  

4.7 

Resembles  hematite. 

Hausmannite,p.  282.  .  . 

4.8 

Chestnut  -brown 

streak. 

MAGNETITE,  p.  279. 

5.1 

Strongly  magnetic. 

Franklinite,  p.  280  

5.1 

Associated  with  wille- 

mite  and  zincite. 

HEMATITE,  p.  270.  .. 

5.2 

Red-brown  streak. 

Columbite,  p.  312:  

5.6 

Submetallic  luster. 

Wolframite,  p.  338  

7.4 

Submetallic  luster. 

CASSITERITE,  p.  272 

7.5 

Adamantine  luster. 

Pitchblende,  p.  324.... 

7.5 

Pitchy  luster. 

31 


482    INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


2.  Color  tin-white  to  light  gray. 


Sp.  gr. 

Remarks 

(a)  The    mineral    is    STIBNITE,  p.  225  
scratched   by  the    Molybdenite,  p.  227.  .. 
finger-nail. 

4.5 

4.7 

Lead-gray  streak. 
Greenish    streak    on 
glazed  paper. 

(6)  The    mineral    is    Iron,  p.  223  

7  0 

Strongly  magnetic 

not  scratched   b  y    GALENA,  p.  228  
the  finger-nail,  but 
is    scratched    by  a    SILVER,  p.  220  
knife  blade.                  Platinum,  p.  222 

7.5 

10.5 
15  0- 

Granular    structure 
due  to  cleavage. 
Usually  tarnished. 
In  grains  scales  and 

19.0 

nuggets. 

(c)  The    mineral    is    ARSENOPYRITE,    p. 
not  scratched  by  a       240. 
knife  blade.                  Smaltite,  p.  238  

6.0 

6.2 

Usually  occurs  with 
pyrite,  galena,  etc. 
Usually  occurs  with 

silver  minerals. 

3.    Color  brass,  bronze^  red,  brown,  etc. 


(a)  The    mineral    is 
scratched    by    the 
finger-nail. 

(6)  The    mineral    is    CHALCOPYRITE,    p. 
not    scratched    b  y       244 

4  2 

Brass  yellow 

the  finger-nail,  but    PYRRHOTITE,  p.  235  . 
is    scratched   by   a    Pentlandite,  p.  233  .... 
knife  blade.                  BORNITE,  p.  245  

Cuprite,  p.  267 

4.6 
4.8 
5.1 

6.0 

Somewhat  magnetic. 
Non-magnetic. 
Peculiar      purple- 
brown.     Easily  tar- 
nished. 
Deep  red  translucent. 

COPPER,  p.  221  

8.8 

Characteristic  color. 

Calaverite,  p.  242  
GOLD,  p.  218  

8.8 
15.0- 
19  0 

Brittle. 
Malleable. 

(c)  The    mineral    is    Rutile,  p.  273.  .  
not   scratched    b  y    Mar  ca  site,  p   239. 

4.2 

4  9 

Brownish  red. 
Resembles  pyrite. 

a  knife  blade.              PYRITE,  p.  236  

5.0 

Brass  yellow. 

THE  DETERMINATION  OF  MINERALS 


483 


II.  LUSTER  ADAMANTINE  TO  SUB -ADAMANTINE. 
(Brilliant,  but  transparent  or  translucent  on  thin  edges.) 


Sp.  gr. 

Remarks 

(a)  The    mineral    is 

SULFUR,  p.  217. 

2.0 

Brittle. 

scratched    by   the 

Cerargyrite,  p.  253  

5.5 

Very  sectile. 

finger-nail. 

(6)  The    mineral    is 

SPHALERITE,  p.  231  . 

4.0 

Usually  shows  good 

not  scratched    by 

cleavage. 

the  finger-nail,  but 

Pyrargyrite,  p.  246  

5.8 

Red  on  thin  edges. 

is    scratched   by    a 

Cuprite,  p.  267  

6.0 

Deep  red  translucent. 

knife  blade. 

Scheelite,  p.  339  

6.0 

Sub-adamantine   lus- 

ter. 

Anglesite,  p.  329  

6.3 

Resembles  cerussite. 

CERUSSITE,  p.  305... 

6.5 

Colorless,     white    or 

pale  colors. 

Wulfenite,  p.  340  

6.7 

Usually  yellow  to 

red. 

Pyromorphite,  p.  315.  . 

6.8 

Usually    green    or 

brown. 

Vanadinite,  p.  317  

6.8 

Usually  red. 

Mimetite,  p.  316 

7.2 

Resembles    v  a  n  a  d  - 

inite. 

CINNABAR,  p.  234.  .  .  . 

8.0 

Vermilion-red. 

(c)  The    mineral    is 

Diamond,  p.  213  

3.5 

Loose  crystals.     Oc- 

not scratched  by  a 

tahedral  cleavage. 

knife  blade. 

Spinel,  p.  278 

3.6 

Usuallv  in  octahedral 

crystals  or  grains. 

CORUNDUM,  p.  268.. 

4.0 

More  or  less  perfect 

hexagonal  crystals. 

Rutile,  p.  273  

4.2 

Reddish  brown. 

Zircon,  p.  379  

4.6 

Usually  brown. 

CASSITERITE,  p.  272 

7.5 

Very  heavy. 

484        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


HI.  LUSTER  NON-METALLIC  (AND  NOT  ADAMANTINE). 
1.  Color  red  or  pink. 


(a)  The    mineral   is 

GYPSUM,  p.  332. 

2  3 

Crystalline  granular. 

scratched  by  the 
finger-nail. 

CLIACHITE,  p.  287.  .. 
HEMATITE,  p.  270.  .. 

2.5 
5.2 

Usually  pisolitic. 
Clay-like. 
Earthy  structure. 

(b)  The  mineral  is 
not  scratched  b  y 
the  finger-nail,  but 
is  scratched  by  a 

Chabazite,  p.  262  

OPAL,  p.  262  
Alunite,  p.  335  

2.1 

2.1 

2.6 

Cube-like  rhombohe- 
drons. 
Luster  —  greasy. 
Difficult  to  recognize 

knife  blade. 

CALCITE,  p  291 

2  7 

at  sight. 
Usually  shows  cleav- 

DOLOMITE, p.  296... 
Lepidolite,  p.  395  
ANHYDRITE,  p.  330.. 
Magnesite,  p.  298  

Rhodochrosite,  p.  300. 
BARITE,  p.  327  

2.8 
2.8 
2.9 
3.1 

3.5 

4.5 

age. 
Resembles  calcite. 
Lilac-colored  scales. 
Resembles  limestone. 
Rhombohedral  cleav- 
age. 
Usually  shows  rhom- 
bohedral  cleavage. 
Very  heavy. 

(c)  The  mineral  is 
not  scratched  by  a 
knife  blade. 

OPAL,  p.  262  
CHALCEDONY,  p.  261 
QUARTZ,  p.  258  
Beryl,  p.  368  
Andalusite,  p.  381  

Chondrodite,  p.  402.  .  .  . 
TOURMALINE,  p.  388 
Rhodonite,  p.  362  

2.1 
2.6 
2.6 

2.7 
3.1 

3.1 
3.1 
3.6 

Luster  greasy. 
Luster  dull. 
Luster  vitreous. 
Resembles  quartz. 
Usually   has    colum- 
nar structure. 
Usually  in  crystalline 
limestone. 
Usually  in  prismatic 
crystals. 
Resembles     r  h  o  d  o  - 

• 

GARNET,  p.  371  
Willemite,  p.  375  

3.8- 
4.2 
4.1 

chrosite. 
May    show    granular 
structure. 
Usually  with  frank- 
lin ite. 

THE  DETERMINATION  OF  MINERALS 


485 


2.  Color  yellow. 


(a)  The    mineral    is 

Sp.  gr. 

Remarks 

finger-nail. 

GYPSUM,  p.  332  
Carnotite,  p.  321  

2.3 
4.1 

Crystalline  granular. 
In  sandstone. 

(6)  The  mineral  is 
not  scratched  b  y 
the  finger-nail,  but 
is  scratched  by  a 
knife  blade. 

OPAL,  p.  262  
ANTIGORITE,  p.  398. 
CALCITE,  p.  291  

COLLOPHANE,  p.  319 
Jarosite,  p.  336  
LIMONITE,  p.  286.... 
SMITHSONITE,     p. 
301 

2.1 
2.5 
2.7 

2.8 
3.2 
3.8 

4  4 

Greasy  luster. 
Compact;  dull  luster. 
Usually  shows  cleav- 
age. 
Amorphous,  no  cleav. 
Earthy. 
Yellow-brown  streak. 

Curved  cleavage. 

Stibiconite,  p.  275  

5.2 

Pale  yellow. 

(c)  The  mineral  is 

OPAL,  p.  262... 

2  1 

Greasy  luster. 

not  scratched  by  a 
knife  blade. 

CHALCEDONY,  p.  261 
QUARTZ,  p.  258  

2.6 
2.6 

Dull  luster. 
Vitreous  luster. 

Beryl,  p.  368  
GARNET,  p.  371     . 

2.7 
3  8 

Resembles  quartz. 
No  cleavage  or  struc- 

Willemite, p.  375  

4.1 

ture. 
Usually  with  frank- 
linite. 

3.  Color  green. 


(a)  The  mineral  is 
scratched  by  the 

Glauconite,  p.  406  
Garnierite,  p.  404 

2.3 
2  5 

Usually  in  grains. 
Massive  apple-green 

finger-nail. 

TALC,  p.  400  
CHLORITE,  p.  397.  .. 

Cerargyrite,  p.  253  

2.7 
2.8 

5.5 

Pale  green. 
Scaly.    Light  to  dark 
green. 
Very  sectile. 

(b)  The  mineral  is 
not  scratched  by 
the  finger-nail,  but 
is  scratched  by  a 
knife  blade. 

OPAL,  p.  262  
CHRYSOCOLLA,   p. 
405  
ANTIGORITE,  p.  398 
APATITE,  p.  313  
Brochantite,  p.  332  
MALACHITE,  p.  307.. 
SMITHSONITE,     p. 
301. 

2.1 

2.1 
2.5 
3.2 
3.9 
3.9 
4.4 

Greasy  luster. 

Bluish-green. 
Usually  compact. 
No  cleavage. 
Resembles  malachite. 
Usually  fibrous. 
Curved     rhombohe- 
dral  cleavage. 

Pyromorphite,  p.  315.  . 

6.8 

Occurs   with   lead 
minerals. 

486        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Sp.  gr. 

Remarks 

(c)  The    mineral    is 

OPAL,  p.  262.... 

2  1 

Greasy  luster 

not  scratched  by  a 
knife  blade. 

CHALCEDONY,  p.  261 
Beryl,  p.  368  
Turquois,  p.  320  

Prehnite,  p  387 

2.6 

2.7 
2.7 

2  9 

Dull  luster. 
Prismatic  crystals. 
Bluish  green.  Appar- 
ently amorphous. 
With  zeolites 

TOURMALINE,  p.  388 
Diopside,  p.  360  

3.1 
3.2 

Prismatic  crystals. 
In  limestones. 

Forsterite,  p.  374  
OLIVINE,  p.  373  
Clinozoisite,  p.  386.  .  .  . 
EPIDOTE,  p.  384  
Vesuvianite,  p.  378  .  .  . 
GARNET,  p  371. 

3.2 
3.3 
3.3 
3.4 
3.4 
3  8  — 

Granular. 
Granular. 
Pale  green. 
Pistache  green. 

May  be  granular 

Willemite,  p.  375  

4.2 
4.1 

With   franklinite. 

4.  Color  blue. 


(a)  The  mineral  is 
scratched  by  the 
finger-nail. 

Chalcanthite,  p.  334  ... 
Vivianite,  p.  318  

2.2 
2.6 

Soluble   in   water 
(taste). 
Cleavable  or  earthy. 

(t)  The  mineral  is 
not  scratched  by 
the  finger-nail,  but 

CHRYSOCOLLA,  p. 

405  
Lazurite,  p.  357  

2.1 

2.4 

Greenish  blue. 
Dark  blue.    Withpy- 

is  scratched  by  a 
knife  blade. 

Lepidolite,  p.  395  

CALCITE,  p.  291  
ANHYDRITE,  p.  330.. 
Azurite,  p.  307  

2.8 

2.7 
2.9 
3  8 

rite.     (lapis  lazuli) 
Scaly.    With  tourma- 
line. 
Shows  cleavage. 
Resembles  limestone. 
Deep  blue. 

Celestite,  p.  328  

3.9 

Resembles  barite. 

(c)  The  mineral  is 
not  scratched  by  a 
knife  blade. 

Sodalite,  p.  356  
CHALCEDONY,  p.  261 

Beryl,  p.  368 

2.3 

2.6 

2  7 

With  feldspars. 
Compact,  apparently 
amorphous. 
Prismatic  crystals. 

- 

Turquois,  p.  320  
Glaucophane,  p.  367.  .  . 

TOURMALINE,  p.  388 
CORUNDUM,  p.  268.. 

2.7 
3.1 

3.1 
4.0 

Greenish  blue. 
Dark    blue   to   blue- 
black. 
Prismatic  crystals. 
Usually   bluish-gray. 
Very  hard. 

THE  DETERMINATION  OF  MINERALS 


487 


6.  White,  colorless,  or  nearly  so  (pale  tints  of  any  hue  are  in- 
cluded here). 


Sp.  gr. 

Remarks 

(a)  The  mineral  is 
scratched  by  the 

Carnallite,  p.  257  
Ulexite,  p.  324  

1.6 
1.6 

Soluble  in  water. 
Shows  minute  fibres. 

finger-nail. 

Sylvite,  p.  253  
Hydromagnesite,  p.  308 
Halloysite,  p.  404  
GYPSUM,  p.  332  
Nitratine,  p.  322  
Brucite,  p.  288  
CLIACHITE,  p.  287... 

Kaolinite,  p.  403  
TALC,  p.  400  

2.0 
2.1 
2.2 
2.3 
2.3 
2.4 
2.5 

2.6 

2.7 

Soluble  in  water. 
Usually  in  serpentine. 
Clay-like. 
Often  granular. 
Soluble  in  water. 
Difficult  to  recognize. 
Usually    pisolitic. 
Clay-like. 
Clay-like. 
Soapy  feel. 

Sericite,  p.  394  

2  8 

Scaly,    in    part    like 

Pyrophyllite,  p.  401  

2.8 

talc. 
Resembles  talc. 

(6)  The  mineral  is 
not  scratched  by 

HALITE,  p.  251  
Kainite,  p.  331 

2.1 
2  1 

Soluble  in  water. 
Soluble  in  water. 

the  finger-nail,  but 
is  scratched  by  a 
knife  blade. 

ZEOLITE,  p.  407  
OPAL,  p.  262 

2.1- 
2.3 
2  1 

Crystalline.      Varia- 
ble. 
Amorphous. 

Hydromagnesite,  p.  308 
Colemanite,  p.  323  
Gibbsite,  p.  286  
CALCITE,  p.  291  

DOLOMITE,  p.  296... 
COLLOPHANE,  p.  319 
Wollastonite,  p.  369.  .  . 
ANHYDRITE,  p.  330.. 
Aragonite,  p.  302  

2.1 

2.4 
2.4 

2.7 

2.8 
2.8 
2.8 
2.9 
2  9 

Usually  in  serpentine. 
Difficult  to  recognize. 
Usually  in  bauxite. 
Usually  good   cleav- 
age. 
Resembles  calcite. 
Difficult  to  recognize. 
Columnar  or  fibrous. 
Resembles  limestone. 
Cleavage    imperfect 

Cryolite,  p.  256.  .. 

3  0 

or  lacking. 
Almost  disappears  in 

Dahllite,  p.  315 

3  0 

water. 
Usually  an  incrusta- 

Magnesite, p.  298  
FLUORITE,  p.  254.... 

3.1 
3.2 

tion. 
Resembles  porcelain. 
Shows  cleavage. 

488         INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


Sp.  gr. 

Remarks 

CALAMINE,  p.  376.  .. 

Strontianite,  p.  304  
Celestite,  p.  328 

3.4 

3.7 
3  9 

Sub  radiating    struc- 
ture. 
Resembles  aragonite. 
Resembles  barite 

Witherite,  p.  305  
SMITHSONITE,p.301 
BARITE,  p.  327. 

4.3 
4.4 
4  5 

Very  heavy. 
Curved  cleavage. 
Usually  shows  platy 

Stibiconite,  p.  275  
Scheelite,  p.  339  .  .  .  . 

5.2 
6  0 

structure. 
Yellowish  white. 
Subadamantine    lus- 

ter. 

(c)  The    mineral    is    OPAL,  p.  262  
not  scratched  by  a 
knife  blade.                 CHALCEDONY,  p.  261 
QUARTZ,  p.  258  
Adularia,  p.  346  
Microcline,  p.  347  
ORTHOCLASE,  p.  344 
PLAGIOCLASE,  p.  348 

Nepheline,  p.  355  
Beryl,  p.  368  
Scapolite,  p.  377.  . 

2.1 

2.6 
2.6 
2.6 
2.6 
2.6 
2.6 

2.6 

2.7 
2  7 

Greasy  luster.     Hya- 
lite, vitreous. 
Dull  luster. 
Vitreous  luster. 
Vein  mineral. 
Like  orthoclase. 
No  twin  striations. 
Usually    with     twin 
striations. 
Greasy  luster. 
Resembles  quartz. 
More  or  less  colum- 

Datolite, p.  391  
Spodumene,  p.  370.  .  .  . 
Diopside,  p.  360  

2.9 
3.1 
3.2 

nar. 
Colorless  to  greenish 
white. 
Shows  more  or  less 
cleavage. 
White  to  pale  green. 

Forsterite,  p.  374  
GARNET,  p.  371  

3.2 

3.8- 
4.2 

Difficult  to  recognize. 
Often  granular. 

THE  DETERMINATION  OF  MINERALS 


489 


6.  Color  gray. 


Sp.  gr. 


Remarks 


(a)  The  mineral  is 
scratched  by  the 
finger-nail. 

GYPSUM,  p.  332  
Cerargyrite,  p.  253  

2.3 
5.5 

Usually  granular. 
Very  sectile. 

(&)  The  mineral  is 
not  scratched  by 
the  finger-nail,  but 
is  scratched  by  a 
knife  blade. 

OPAL,  p.  262:  
Gibbsite,  p.  286  
CALCITE,  p.  291  

DOLOMITE,  p.  296.  .  . 
COLLOPHANE,p.  319 

ANHYDRITE,  p.  330.. 
Magnesite,  p.  298 

2.1 
2.4 

2.7 

2.8 
2.8 

2.9 
3  1 

Amorphous. 
Occurs  in  bauxite. 
Usually  shows  cleav- 
age. 
Resembles  calcite. 
Amorphous.     No 
cleavage. 
Resembles  calcite. 
Resembles  calcite. 

SIDERITE,  p.  299.... 
SMITHSONITE,    p. 
301. 
BARITE,  p.  327  

3.8 
4.4 

4.5 

Too  heavy  for  calcite. 
Usually  shows  curved 
cleavage. 
Usually  more  or  less 

Anglesite,  p.  329  

6.3 

platy. 
Very    heavy.     Asso- 
ciated  with    lead 
minerals. 

(c)  The    mineral    is 

OPAL,  p.  262. 

2  1 

Greasy  luster 

not  scratched  by  a 
knife  blade. 

CHALCEDONY,  p.  261 

QUARTZ,  p.  258  
ORTHOCLASE,  p.  344 

PLAGIOCLASE,  p.  348 
Nepheline,  p.  355. 

2.6 

2.6 
2.6 

2.6 
2  6 

Dull  luster.      Com- 
pact. 
Vitreous  luster. 
Usually  shows  cleav- 
age. 
Usually  shows  cleav- 
age and  twin  stria- 
tions. 
Greasy  luster 

Scapolite,  p.  377  

2.7 

More  or  less  colum- 

Andalusite, 381  
Spodumene,  p.  370.  .  .  . 
Forsterite,  p.  374  
Clinozoisite,  p.  386.  .  .  . 

CORUNDUM,  p.  268. 

3.1 
3.1 
3.2 
3.3 

4.0 

nar. 
Associated  with  mica. 
Shows  cleavage. 
Granular. 
Yellowish  or  greenish 
gray. 
Shows  parting. 

490        INTRODUCTION  TO  THE  STUDY  OF  MINERALS 


7.  Color  brown. 


Sp.  gr. 


Remarks 


(a)  The    mineral   is 

GYPSUM,  p.  332  

2.3 

Crystalline. 

scratched   by  the 

CLIACHITE,  p.  287... 

2.5 

Usually    pisolitic. 

finger-nail. 

Amorphous. 

LIMONITE,  p.  286.  .  .  . 

3.8 

Amorphous.   Yellow- 

brown  streak. 

(6)  The    mineral    is 

OPAL,  p.  262          

2.1 

Greasy  luster. 

not    scratched    by 

ANTIGORITE,  p.  398. 

2.5 

Compact. 

the  finger-nail,  but 

CALCITE,  p.  291  

2.7 

Too  light  for  siderite. 

is   scratched  by   a 

COLLOPHANE,  p.  319 

2.8 

Difficult  to  recognize. 

knife  blade. 

APATITE,  p.  313  

3.2 

Reddish  brown. 

Jarosite,  p.  336  

3.2 

Yellow  to  brown. 

LIMONITE,  p.  286.... 

3.8 

Amorphous.   Yellow- 

brown  streak. 

SIDERITE,  p.  299  

3.8 

Rhombohedral  cleav- 

age like  calcite. 

SPHALERITE,  p.  231. 

4.0 

Usually  shows  cleav- 

age. 

PSILOMELANE,   p. 

289  

4.2 

Earthy  and  dull. 

Goethite,  p.  284  

4.3 

Yellow-brown  streak. 

(c)  The    mineral    is 

OPAL,  p  262 

2.1 

Greasy  luster. 

not  scratched  by  a 

CHALCEDONY,  p.  261 

2.6 

Dull  luster. 

knife  blade. 

TOURMALINE,  p.  388 

3.1 

In    crystalline    lime- 

stone. 

Vesuvianite,  p.  378...  . 

3.4 

Usually  columnar. 

Titanite,  p.  412  .,. 

3.5 

Adamantine  luster. 

Staurolite,  p.  387  

3.7 

Prismatic  crystals. 

GARNET,  p.  371  

3.8- 

No    cleavage.     May 

4.2 

be  granular. 

Rutile,  p.  273  

4.2 

Reddish  brown. 

CASSITERITE,p.272. 

7.5 

Very  heavy. 

THE  DETERMINATION  OF  MINERALS 


491 


8.  Black.     (Very  dark  shades  of  any  color  are  included  here.) 


Sp.  gr. 

Remarks 

(a)  The    mineral   is 

GRAPHITE,  p.  216.... 

2.1 

Earthy  and  dull. 

scratched   by   the 

PSILOMELANE,   p. 

finger-nail. 

289 

4.2 

Dull  luster. 

Pyrolusite,  p.  274  

4.8 

Usually  more  or  less 

fibrous. 

(fc)  The    mineral    is 

OPAL,  p.  262  

2.1 

Greasy  luster. 

not  scratched  by 

ANTIGORITE,  p.  398. 

2.5 

Usually    greenish- 

the  finger-nail,  but 

black. 

is   scratched  by   a 

CALCITE,  p.  291 

2.7 

Usually  shows  cleav- 

knife blade. 

age. 

COLLOPHANE,  p.  319 

2.8 

Often  oolitic. 

SPHALERITE,  p.  231. 

4.0 

Good  cleavage. 

PSILOMELANE,    p. 

289 

4.2 

Dull  luster. 

(c)  The    mineral    is 

OPAL,  p.  262. 

2.1 

Greasy  luster. 

not  scratched  by  a 

CHALCEDONY,  p.  261 

2.6 

Dull  luster 

knife  blade. 

QUARTZ,  p.  258  

2.6 

Vitreous  luster. 

TOURMALINE,  p.  388 

3.1 

No  cleavage. 

PYROXENE,  p.  359... 

3.2 

Imperfect  cleavage. 

Hypersthene,  p.  359.  .  . 

3.4 

Lamellar  structure. 

Spinel,  p,  278  

3.6 

Usually    in    octahe- 

dral crystals. 

LIMONITE,  p.  286.... 

3.8 

Yellow-brown  streak. 

GARNET,  p.  371  

3.8- 

Usually  in  crystals. 

PSILOMELANE,  p. 

4.2 

289  

4.2 

Dull  luster. 

Rutile,  p.  273  

4.2 

Difficult  to  recognize. 

CASSITERITE,  p.  272  . 

7.5 

Very  heavy. 

SYNOPSIS  OF  TABLE  II 

A.  MINERALS  WITH  NON-METALLIC  LUSTER. 

(If  the  luster  is  in  doubt  it  may  be  disregarded,  in  which  case  the  sulfids 
of  division  7  should  be  included  with  division  8  and  the  remainder  of  divi- 
sion 7  with  those  of  divisipn  9.) 

1.  Carbonates  (the  mineral  effervesces  in  HC1). 

2.  Sulfates  (the  water  solution  of  the  Na2CO3  fusion  gives  a  white  ppt. 

with  BaCl2). 

3.  Silicates  (fragments  are  insoluble  in  a  NaPO3  bead). 

4.  Phosphates   and   Arsenates    (the   HNO3   solution   of  the    Na2CO3 

fusion  gives  a  yellow  ppt.  with  (NH4)2MoO4). 
6.  Chromates,  Vanadates,  and  Molybdates  (green  NaPO3  bead  in  R.F.). 

6.  Chlorids  (NaPO3  bead  with  CuO  gives  blue  flame). 

7.  Not  previously  included. 

B.  MINERALS  WITH  METALLIC  OR  SUB -METALLIC  LUSTER. 

8.  Sulfids  and  Sulfo-salts  (the  Na2CO3  fusion  made  on  mica  O.F.  stains 

a  moistened  silver  coin). 

9.  Not  previously  included. 


492 


DETERMINATION  OF  MINERALS 


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THE  DETERMINATION  OF  MINERALS  503 

BLANK  FORM  FOR  REPORTING  MINERALS 

Date No Name 

Form 

Cleavage 

Luster  and  color 

Hardness 

Streak 

Spec.  Grav 

Other  characters 

Associates Mineral    suspected 

Optical  characters  of  crushed  fragments 


Fusibility 

Flame  coloration 

Closed  tube 

Open  tube 

On  charcoal  alone 

On  charcoal  with  Na2COs 

Borax  bead 

NaPO*  bead 

Solubility 


Wet  tests 

Miscellaneous  tests 


Summary  of  important  characters 

Mineral 

*  The  space  in  the  upper  right-hand  corner  is  for  a  sketch  of  crystals  or 
crushed  fragments. 


INDEX  AND  GLOSSARY 


An.  Symbol  used  for  an  axis  of 
n-fold  symmetry. 

Abrasives.  Corundum,  diamond, 
diatomite,  emery,  garnet, 
quartz. 

Absorption,  201 
scheme,  204 

Accessory  minerals  of  igneous  rocks, 
424 

Acicular.     Needle-shaped. 

"Acid"  igneous  rocks.  The  same 
as  persilicic  rocks,  423 

Acids,  8 

Acid  salt,  10 

Actinolite.     Ferriferous  tremolite. 

Acute  bisectrix,  193 

Adamantine  luster,  154 

Adjustments  of  the  polarizing  micro- 
scope, 173 

Adularia,  346 

Agate.  A  banded  or  variegated 
variety  of  chalcedony. 

Agglomerate,  439 

Aggregate  polarization.  The  intri- 
cate structure  of  microcrystal- 
line  substances  between  crossed 
nicols. 

Alabaster.  Translucent  massive 
gypsum.  Formerly  used  for 
onyx  marble. 

Alaskite,  429 

Albite,  350 

twinning,  349 


Allotriomorphic.  The  same  as  an- 
hedral. 

Almandite,  371 

Alpha  (a).  (1)  The  angle  between 
the  b-  and  c-axes  of  reference  in 
the  triclinic  system.  (2)  The 
direction  of  the  fastest  ray  in 
anisotropic  crystals. 

Alteration  of  minerals,  420 

Alumina.     Aluminum  oxid  (A12O3). 

Aluminates,  277 

Aluminum  minerals.  Alunite,  and- 
alusite,  cliachite,  corundum 
cryolite,  gibbsite,  halloysite 
kaolinite,  kyanite,  sillimannite, 
topaz,  turquois,  and  many 
silicates, 
tests  for,  40 

Alundum.  Artificial  A12O3  used  as 
an  abrasive. 

Alunite,  335 

Amazon-stone.  The  green  variety 
of  microcline. 

Amber,  415 

Amethyst.  The  purple  variety  of 
quartz  used  as  a  gem. 

Amorphous  condition,  54 

Amphibole.  A  group  of  silicates 
including  anthophyllite,  tremo- 
lite (actinolite),  hornblende, 
and  glaucophane,  363 

Amygdaloidal,  426 

Analcite,  410 

Analyzer,  173 

Andalusite,  381 


505 


506 


INDEX  AND  GLOSSARY 


Andesine,  352 
Andesite,  433 
Andradite,  371 
Angle,  axial,  192 

critical,  161 

extinction,  185 

interfacial,  56 
Anglesite,  329 
Anhedral,  54 
ANHYDRITE  mineral,  330 

rock,  446 
Anisotropic,  190 
Ankerite.     Ferriferous  dolomite. 
Anomalous  optical  properties,  206 
Anorthite,  354 
Anthophyllite,  364 
Anthracite,  416 
ANTIGORITE,  398 
Antimony      glance.     Synonym      of 

stibnite. 

minerals,  Jamesonite,  polyba- 
site,  pyrargyrite,  stephanite, 
stibnite,  stibiconite,  tetra- 
hedrite.  tests  for,  41 

ocher,  Synonym  of  stibiconite. 
£>n.     Symbol    used    for  composite 

symmetry  with   respect  to  an 

axis  of  ?i-fold  symmetry  and  a 

plane  normal  thereto. 
APATITE,  313 
Apophyllite,  406 
Apparatus,  blowpipe,  20 
Aquamarine.     A    transparent    sea- 
green  variety  of  beryl. 
Aragonite,  302 

Arborescent.     Branching  like  a  tree. 
Argentite,  228 
Arkose,  441 
Arsenates,  310 
Arsenic        minerals.     Arsenopyrite, 

enargite,     mimetite,     smaltite. 

tests  for,  41 


ARSENOPYRITE,  240 

Asbestos.  Fibrous  varieties  of  tre- 
molite,  anthophyllite,  or 
chrysotile. 

Asbolite.  A  soft  earthy  cobalt- 
bearing  variety  of  psilomelane. 

Asphaltum,  415 

Association  of  minerals,  417 

Asterism.  A  six-rayed  star  effect 
produced  by  symmetrically  ar- 
ranged inclusions  in  certain 
minerals. 

Asymmetric  class.  The  crystal 
class  devoid  of  symmetry. 

Atomic  weights,  table  of,  6 

Auganite,  434 

porphyry,  428 

Augite,  361 

Augitite,  436 

Automorphic.  The  same  as 
euhedral. 

Aventurine.  A  spangled  appear- 
ance produced  by  inclusions  of 
hematite,  goethite,  or  mica. 

Axes  of  reference,  73 
of  symmetry,  60 
optic,  191 

Axial  angle,  192 
colors,  202 
cross,  85 

elements.     A    collective    name 
used  for  the  axial  ratio  and 
the  angles  between  the  axes 
of  reference, 
plane,  192 
ratio,  75 

Axinite,  391 

Azurite,  307 

B 

Balance,  specific  gravity,  149 
BARITE,  327 


INDEX  AND  GLOSSARY 


507 


Barium  minerals.     Barite,  witherite. 

tests  for,  42 

Barytes.     Synonym  of  barite. 
Basal       pinacoid.     The       pinacoid 

{001}  or  j 0001 | 
Basalt,  435 

porphyry,  428 
Basaltic  hornblende.     A  deep  brown 

variety  of  hornblende  found  in 

volcanic  rocks. 
Bases,  9 
"Basic"  igneous  rocks.     The  same 

as  subsilicic  rocks,  423 

salt,  10 
Bastite.     Lamellar  antigorite  pseu- 

domorphous  after  pyroxene. 
Batholith,  426 
Bauxite,  442 
Baveno  twin,  345 
Bead  tests,  31-32 
Becke,  Austrian  mineralogist  (1855- 


Becke  test,  165 

Berlin  blue.  An  anomalous  inter- 
ference color  of  the  first-order. 

Bertrand  lens. 

Beryl,  368 

Beryllium  minerals.     Beryl, 
tests  for,  42 

Beta  (0).  (1)  The  angle  between 
the  a-  and  c-  axes  of  reference  in 
the  monoclinic  and  triclinic 
systems.  (2)  The  direction  nor- 
mal to  the  plane  of  a  and  7  in 
biaxial  crystals. 

Biaxial,  191. 

Bibliography  XV-XVII1. 

BIOTITE,  396 

Bipyramid,  69 

Birefringence.  The  strength  of  the 
double  refraction. 

Bisectrix,  193 


Bisilicates.      The    same    as     meta- 

silicates    RnSiO3    =    RO.SiO2, 

343 
Bismuth  flux.     The  same  as  iodid 

flux,  23 

minerals.     Bismuthinite. 

tests  for,  42 
Bismuthinite,  226 
Bisphenoid,  rhombic,  71 

tetragonal,  71 

Blackband.     An     impure     carbon- 
aceous siderite. 
Blackjack.     A    miner's    name    for 

ferriferous  sphalerite. 
Black    opal.     A    dark-colored    pre- 
cious opal. 
Bladed,  128 

Blende.     Synonym     of     sphalerite. 
Blowpipe,  20 

apparatus,  20 

reagents,  22 

tests,  25 
Bluestone.     Synonym    of    chalcan- 

thite. 

Bog  iron  ore.     Synonym  of  limonite. 
Bog  manganese.     Synonym  of  wad, 

an   impure    earthy    variety    of 

psilomelane. 
Bone-ash  (reagent),  23 
Bone     turquois.        An     aluminous 

variety   of   cellophane    formed 

by   the    replacement    of    fossil 

teeth. 

Borates,  310 
Borax  (reagent),  22 

bead  tests,  31 
Boric  acid  flux,  23 
BORNITE,  245 
Boron    minerals.     Axinite,    colema- 

nite,  datolite,  tourmaline,  ule- 

xite. 

tests  for,  42 


508 


INDEX  AND  GLOSSARY 


Bort.  Diamond  in  the  form  of 
aggregates  without  distinct 
cleavage. 

Botryoidal,  128 

Bravais.  French  crystallographer 
and  physicist  (1811-63). 

Brazilian  twin  of  quartz.  Inver- 
sions twins  of  a  right-handed 
crystal  with  a  left-handed  one. 

Breccia,  441 

Breithaupt,  German  mineralogist 
(1791-1873). 

Brittle  silver  ore.  Synonym  of 
stephanite. 

Brochantite,  332 

Bronze  mica.  Synonym  of  phlogo- 
pite. 

Bronzite.  Ferriferous  variety  of 
enstatite. 

Brucite,  288 

Brush,  American  mineralogist  (1831- 
1912). 


C.  Symbol  used  to  indicate  that  a 
crystal  has  a  center  of  symmetry. 

Cabochon.  The  rounded  form  of 
cut  gem-stones  used  especially 
for  opal. 

Cairngorm.     Smoky  quartz. 

CALAMINE,  376 

Calaverite,  242 

Calcareous  ooze,  444 
tufa,  448 

Calc-spar.     Synonym  of  calcite. 

CALCITE,  291 

Calcium  minerals.  Anhydrite,  apa- 
tite, aragonite,  calcite,  clinozoi- 
site,  colemanite,  cellophane, 
dahllite,  dolomite,  fluorite,  gyp- 
sum, scheelite,  wollastonite, 


ulexite,     and    many    silicates. 

tests  for,  43 
Cane  sugar,  80 
Capillary.     Very    fine     hair- like 

crystals. 

Carbonaceous  rocks,  444 
Carbonado.     A  black  tough  opaque 

variety    of    diamond    used    in 

diamond  drills. 
Carbonates,  290 

tests  for,  43 
Carborundum.     An  artificial  silicon 

carbid  (SiC)  extensively  used  as 

an  abrasive. 
Carbuncle.     Garnet  cut  in  cabochon 

form. 

Carlsbad  twin,  345 
Carnallite,  257 
Carnelian.     A  clear  red  chalcedony 

used  as  a  semi-precious  stone. 
Carnotite,  321 
CASSITERITE,  272 
Cat's  eye.     Gems   which   exhibit   a 

peculiar    reflection    because    of 

their  fibrous  structure. 
Celestite,  328 
Center  of  symmetry,  62 
Cerargyrite,  253 
CERUSSITE,  305 
Ceylonite.     An  iron-bearing  variety 

of  spinel. 
Chabazite,  410 
Chalcanthite,  334 
CHALCEDONY,  261 
CHALCOCITE,  230 
CHALCOPYRITE,  244 
Chalcotrichite.     A  capillary  variety 

of  cuprite. 
Chalk,  433 

Chalybite.     Synonym  of  siderite. 
Charcoal.     Use   of,   as   a  blowpipe 

support,  28 


INDEX  AND  GLOSSARY 


509 


Chatoyant.  The  peculiar  optical 
effect  produced  by  cat's  eye. 

Chemical  composition,  5 
compounds,  58 
properties,  5 
tests,  25 
types,  10 

Chert,  449 

Chessylite.  Synonym  of  azurite, 
from  a  locality,  Chessy  in 
France. 

Chiastolite.  A  variety  of  andalusite 
with  symmetrically  arranged 
carbonaceous  impurities. 

Chile  saltpeter.  The  same  as 
nitratine. 

Chlorids,  251 

Chlorin,  tests  for,  44 

CHLORITE,  397 

Chondrodite,  402 

Chondrules.  Rounded  nodules 
with  excentric  fibrous  struc- 
ture characteristic  of  certain 
meteorites. 

CHROMITE,  281 

Chromium  minerals.     Chromite. 
tests  for,  44 

CHRYSOCOLLA,  405 

Chrysolite.     Synonym     of     olivine. 

Chrysoprase.  An  apple-green  vari- 
ety of  chalcedony  used  as  a 
semi-precious  stone. 

Chrysotile,  400 

CINNABAR,  234 

Citric  acid  (reagent),  23 

Classification  of  crystals,  78 
of  minerals,  212 
of  ores,  455 
of  rocks,  422 

Clastic.  Made  up  of  broken  frag- 
ments of  preexisting  rocks. 

Clay,  442 


Cleavage,  129 

CLIACHITE,  287 

Clinographic  drawing  or  projection,. 
85 

Clinozoisite,  386 

Closed  forms,  72 
tube  tests,  28 

Coal,  444 

Cobalt  minerals.     Smaltite. 
tests  for,  44 

Coefficients,  75 

Colemanite,  323 

Colloform.  The  rounded,  more  or 
less  spherical  forms  assumed  by 
amorphous  and  metacolloidal 
minerals  in  open  spaces. 

Colloids,  17 

COLLOPHANE,  319 

Collophanite.  Synonym  of  cello- 
phane. 

Color  of  minerals,  154 

Colors,  interference,  175 

Columnar,  128 

Columbite,  312 

Columbium.  Synonym  of  niobium, 
one  of  the  chemical  elements. 

Combination  of  forms,  55 

Complementary  forms,  72 

Composite  crystals,  123 
symmetry,  62 

Composition-face  of  a  twin  crystal, 
124 

Conchoidal  fracture,  131 

Concretion,  128 

Conglomerate,  441 

Congruent  forms,  72 

Conoscope.  A  polariscope  for  con- 
vergent light. 

Contact  goniometer,  57 
metamorphism,  453 
twins,  124 

COPPER,  221 


510 


INDEX  AND  GLOSSARY 


Copper  glance.  Synonym  of  chal- 
cocite. 

pyrites.  Synonym  of  chalcopy- 
rite. 

Copper  minerals.  Azurite,  bornite, 
brochantite,  chalcanthite,  chal- 
cocite,  chalcopyrite,  chryso- 
colla,  copper,  covellite,  cuprite, 
enargite,  malachite,  tetrahe- 
drite. 

ores     of:    bornite,     chalcocite, 
chalcopyrite,  copper, 
tests  for,  45 

Cornu,  Austrian  mineralogist  (1882 
-1909). 

Cornuite.  An  amorphous  copper 
silicate,  corresponding  to  crys- 
talline chrysocolla. 

CORUNDUM,  268 

Corundum  syenite,  431 

Covellite,  234 

Critical  angle,  161 

Cristobalite,  264 

Crossed  nicols,  173 

Cryolite,  256 

Cryptocrystalline.  Apparently 

amorphous,  but  made  up  of 
very  fine  interlocking  crystals 
(for  example,  chalcedony). 

Crystal,  definition  of,  54 
drawing,  83 
classes,  78 
systems,  81 

Crystals,  classification  of,  78 

Crystalline  aggregates,  128 

Crystalline  limestone,  452 

Crystallites.  Minute  hair-  or  fern- 
like  forms  found  in  volcanic 
glass.  They  are  supposed  to 
represent  incipient  crystals. 

Crystallographic  axes.  The  same  as 
axes  of  reference. 


Crystallography.  The  science  relat- 
ing to  crystals  in  all  aspects, 
often  erroneously  used  for  geo- 
metrical crystallography. 

Cube,  90 

Cupellation,  silver,  34 

Cuprite,  267 

Curve  of  hardness,  154 

Cyanite.     Variant  of  kyanite,  382 

Cyclic  twin,  124 

D 

Dacite,  432 
Dahllite,  315 

Dana,  American  geologist  and  min- 
eralogist (1813-1895). 
Datolite,  391 
Decrepitate.     To  fly  to  pieces  when 

.heated. 

Dedolomitization,  453 
Deltohedron,  94 

Dendritic.     Branching  like  a  tree. 
Dense  igneous  rocks,  427 
Determination  of  minerals,  463 
Devitrification.    The  gradual  change 

of  glass  to  crystalline  aggregates. 
Diabase,  435 
Diallage.     A    lamellar    variety    of 

diopside. 
Diamond,  213 
Diatomaceous  earth.  A  synonym 

of  diatomite. 
Diatomite,  444 
Dichroic,  202 
Dichroscope,  201 
Dihexagonal    bipyramid    class,    102 

pyramidal  class,  80 
Dike,  426 
Dimorphism.     The   particular   case 

of  polymorphism  in  which  there 

are  two  polymorphs. 
Diopside,  360 


INDEX  AND  GLOSSARY 


511 


Diorite,  433 

Diploid,  96 

Diploidal  class,  95 

Dispersion.     The  divergence  of  the 

optical   constants   for   different 

parts  of  the  spectrum. 
Disseminated.     Scattered  through  a 

rock  or  vein  in  small  quantities. 
Disthene.     Synonym  of  kyanite. 
Ditetragonal  bipyramidal  class,  98 

prism,  69 

pyramidal  class,  80 
Ditrigonal  bipyramidal  class,  80 

prism,  69 

pyramidal  class,  107 
Dodecahedron,  90 
Dodecants.     The    twelve    divisions 

into  which  space  is  divided  by 

the  four  axes  of  reference  of  the 

hexagonal  system. 
Dog-tooth    spar.     A      variety      of 

sharp-pointed  calcite  crystals. 
DOLOMITE,  296 
Dolomitic  limestone,  449 
Dolomitization,  449 
Domatic  class.  The  crystal  class  with 

a  single  plane  of  symmetry,  80 
Dome,  69 

Double  refraction,  169 
Drusy.     Apparently  sprinkled  over 

with  minute  crystals. 
Dry-bone.     A    local    synonym     of 

smithsonite  (Wis.)  or  cerussite 

(Mo.) 

Drawing  of  crystals,  83 
Dunite,  436 


Eclogite.  A  metamorphic  rock  with 
garnet  and  either  a  pyroxene 
or  an  amphibole. 

Edge  of  a  crystal,  55 


Effervescence.         The         bubbling 

caused  by  the  evolution  of  gas 

such  as  CO2. 
Efflorescent.     Gives  up 'its  water  of 

crystallization  on  standing. 
Elaeolite.     A  synonym  of  nepheline 

or  nephelite. 
Elastic.     An  elastic  mineral  springs 

back  when  bent  in  contrast  to  a 

flexible  mineral  which  remains 

bent. 
Electrum.     A    naturally    occurring 

alloy  of  gold  or  silver  with  over 

20  per  cent,  of  silver. 
Elements,  axial.     A  collective  name 

used  for  the  axial  ratio  and  the 

angles    between    the    axes    of 

reference. 

list  of  chemical,  6 

occurrence  of  chemical,  6 

of  symmetry,  63 
Ellipse,  optic,  189 
Ellipsoid,  optic,  189 
Emerald.     The  clear  green  variety 

of  beryl  used  as  a  gem. 
Emery.     A    mixture    of    corundum 

with  magnetite. 
Enantiomorphous,  72 
Enargite,  250 
Enstatite,  358 
EPIDOTE,  384 
Equant.     The  same  as  equidimen- 

sional. 

Erubescite.     A  synonym  of  bornite. 
Essonite  (Hessonite).     A  variety  of 

grossularite   garnet   used   as   a 

gem. 

Etch-figures,  66 
Ether,  156 
Euhedral,  54 
Exfoliate.     To  swell  up  or  spread 

out  like  the  leaves  of  a  book. 


512 


INDEX   AND  GLOSSARY 


Extinction,  184 

angle,  185 

direction,  184 
Extraordinary  ray,  169 
Extrusive  (igneous)  rocks,  426 

F 

Fabric,  425 

Face,  crystal,  55 

Face  color.     The  color  of  a  crystal 

when  viewed  in  a  certain  direc- 
tion. 

-symbol,  76 
Facet,  55 
Fahlore.      A    synonym     of     tetra- 

hedrite. 

Faster  ray,  187 

Fedorov,  a  Russian  crystallographer. 
Feldspars,  344 
Feldspathoids,  354 
Felsitic.     The  fine  grained  texture 

of      volcanic      rocks      without 

phenocrysts. 
Ferberite.     The  iron  end-member  of 

the  wolframite  series. 
Ferromagnesian,  424 
Fertilizers,  minerals  used  as:  carnal- 
lite,  collophane,  kainite,  iiitra- 

tine,  sylvite. 
Fire-opal.     Opal       with       fire-like 

reflections. 
Flame,  oxidizing,  25 

reducing,  26 

tests,  26 

Flexible.     See  elastic. 
Flint.     A  massive  chalcedony  rock, 

practically  the  same  as  chert. 
Fluorids,  251 
Fluorin,  tests  for,  45 
Fluorescence,  255 
FLUORITE,  254 

Fluor  spar.     A  synonym  of  fluorite. 
Foliated.     Made  up  of  flat  plates. 


Fool's  gold.     A  synonym  of  pyrite. 
Form,  68 

closed,  72 

general,  79 

limit,  79 

open,  72 

-symbol,  76 
Formula,    determination    of,    of    a 

mineral,  11 

weight.     The  sum  of  the  atomic 
weights  of  the  atoms  of  a 
chemical  compound. 
Forsterite,  374 
Fowlerite.     A   zinc-bearing  variety 

of  rhodonite. 
Fracture,  131 
Franklinite,  280 
Freibergite.     A  silver-bearing  tetra- 

hedrite. 
French    chalk.     A    variety    of    talc 

used  by  tailors. 

Friable.  Capable  of  being  pulver- 
ized by  rubbing  between  the 

fingers. 

Friedel,  G.,  French  crystallo- 
grapher and  mineralogist. 

(1865 —), 

Fusibility,  scale  of,  34 

G 

Gabbro,  434 

GALENA,  228 

Galenite.     A    synonym    of    galena. 

Gamma  (7).  A  Greek  letter  used 
(1)  for  the  angle  between  the 
a-  and  6-axes  of  reference  in  the 
triclinic  system  and  (2)  for  the 
direction  of  the  slowest  ray  in 
anisotropic  crystals. 

Gangue.     The  non-metallic  part  of 
a  vein, 
minerals,  456 

GARNET,  371 


INDEX  AND  GLOSSARY 


513 


Garnierite,  404 

Gel,  17 

Gelatinization,  495 

Gems.     Diamond,    emerald,    ruby, 

sapphire,  etc. 
General  form,  79 
Geode,  128  * 

Geometrical  crystallography,  54 
Geyserite.     A  variety  of  opal  formed 

in  hot  springs. 
Gibbsite,  286 
Gilsonite,  415 
Glance.     An  abbreviation  of  copper 

glance  (chalcocite). 
Glass,  413 
Glauconite,  406 
Glaucophane,  367 
Glide-plane  of  symmetry,    138 
Gliding,  131 
Gneiss,  451 
Goethite,  284 
GOLD,  218 
Gold  minerals.     Calaverite,  gold. 

tests  for,  45 
Goniometer,  contact,  57 

reflection,  57 
Gossan,  459 

Grained  igneous  rocks,  427 
Granite,  429 

porphyry,  430 
Granitic  or  granitoid.     The  texture 

of   even-grained   igneous   rocks 

such  as  granite. 
Granodiorite,  432 
Graphic    determination    of    indices 

and  axial  ratios:  97,   101,   111, 

115,  120,  123. 

granite.  An  intergrowth  of 
quartz  and  orthoclase  or 
microcline. 

texture.     The  texture  like  that 

of  graphic  granite, 
33 


GRAPHITE,  216 

Gray  copper.     A  synonym  of  tetra- 

hedrite. 

Graywacke,  441 
Greisen.     A  quartz-muscovite  rock 

formed     by     high-temperature 

hydrothermal  solutions. 
Greensand,  441 
Greenstone. 
Grossularite,  371 
Groth,  German    crystallographer 

(1843 ). 

Groundmass,    425 

Gypsite,  446 

GYPSUM  (mineral),  332 

(rock),  445 

wedge.     A   thin    wedge-shaped 

slice  of  selenite. 
Gyroid.     A  24-faced  form  with  the 

symmetry  6A2-4A3-3A4. 
Gyroidal  class,  80 


H 


Habit,  crystal.  The  general  shape 
of  a  crystal  determined  by 
growth  in  certain  directions. 

Hackly,  131 

HALITE,  251 

Halloysite,  404 

Haloids,  251 

Hardness,  scale  of,  153 

Hausmannite,  282 

Haiiy,  R.  J.  French  mineralogist 
and  physicist.  The  founder  of 
the  science  of  geometrical  crys- 
tallography (1743-1822). 

Heavy  liquids,  150 

Heavy  spar.     A  synonym  of  barite. 

HEMATITE,  270 

Hematite,  brown.  A  synonym  of 
limonite. 


514 


INDEX  AND  GLOSSARY 


Hemihedral,  83 

Hemimorphic.  Crystals  in  which 
the  two  ends  are  differently 
terminated. 

Hemimorphite.  A  synonym  of 
calamine. 

Hessel.  German  mineralogist 
(1796-1872). 

Heulandite,  408 

Hexagonal    bipyramidal    class,     80 
pyramidal  class,  80 
scalenohedral  class,  104 
system,  102 
trapezohedral  class,  80 

Hexahedron.     Synonym  of  a  cube. 

Hexoctahedral  class,  90 

Hexoctahedron,  91 

Hextetrahedral  class,  93 

Hextetrahedron,  94 

Hiddenite.  An  emerald-green  vari- 
ety of  spodumene  used  as  a  gem. 

Hintze,  German  mineralogist  (1851- 
1916). 

Holohedral,  83 

Holosymmetric,  83 

HORNBLENDE,  365 

Hornblendite.  An  igneous  rock  con- 
sisting essentially  of  hornblende. 

Hornfels,  454 

Horn  silver.  A  synonym  of 
cerargyrite. 

Horse-flesh  ore.  A  synonym  of 
bornite. 

Hiibnerite.  The  manganese  end- 
member  of  the  wolframite  group. 

Hue,  155 

Hyacinth.  A  variety  of  zircon  used 
as  a  gem. 

Hyalite.  A  clear,  colorless,  collo- 
form  variety  of  opal. 

Hydrargillite.  A  synonym  of 
gibbsite. 


Hydrion.     The    element    H    in    an 

acid  salt. 
Hydrocarbons,  414 

tests  for,  44 

Hydrochloric  acid  (reagent),  23 
Hydrogel,  18 
Hydromagnesite,  308 ' 
Hydrothermal  metamorphism,  454 
Hydrous  salts,  11 
Hydroxids,  284 

Hydroxyl.     The  radical  or  ion  (OH). 
Hypersthene,  359 
Hypogene,  461 


Ice  (mineral),  266 

(rock),  449 

Iceland     spar.     The     clear     trans- 
parent    variety     of     cleavable 

calcite. 
Ideal    form.     A    crystal    form    or 

combination  in  which  like  faces 

are  of  the  same  shape  and  size. 
Idocrase.     A       synonym    of   vesu- 

vianite. 

Igneous  rocks,  422 
Ilmenite,  310 
Imitative  forms,  128 
Index  of  refraction,  159 
Indicatrix.     A  synonym  of  the  optic 

ellipsoid. 
Indices,  Miller,  75 

of  refraction,  159 
Indigo     copper.     A     synonym     of 

covellite. 

Injected  rocks,  426 
Intercepts,  73 
Interfacial  angle,  56 
Interference,  175 

colors,  175 

of  light,  175 


INDEX  AND  GLOSSARY 


515 


Interference  figures,  193 

color     chart,     180,     181,     183 

Intergrowth.  An  interlocking  ar- 
rangement of  two  substances 
produced  by  simultaneous  crys- 
tallization. 

Internal  structure  of  crystals,   132 

Intrusive,  426 

Intumesce,  34 

Inversion,  62 
twin,  124 

lodid  flux,  23 

Iridescence.  A  rainbow  effect  pro- 
duced by  the  interference  of 
light  in  thin  surface  films. 

Iron,  223 

minerals.     Arsenopyrite,     bor- 
nite,     chalcopyrite,     chromite, 
columbite,  franklinite,  goethite, 
hematite,    ilmenite,   iron,  jaro- 
site,  limonite,  magnetite,  mar- 
casite,  pyrite,  pyrrhotite,  side- 
rite,   turyite,   vivianite,  wolfra- 
mite, and  some  silicates, 
tests  for,  46 
pyrites.     A  synonym  of  pyrite. 

Isodimorphism,  290 

Isometric  system,  89 

Isomorphism,  14 

Isomorphous  mixtures,  16 

Isotropic,  190 


Jade.  A  tough  compact  green 
or  greenish-white  ornamental 
stone  consisting  either  of  an 
amphibole  (nephrite)  or  of  a 
pyroxene  (jadeite). 

Jamesonite,  245 

Jargon.  A  pale-colored  variety  of 
zircon  used  as  a  gem. 


Jarosite,  336 

Jasper.     A  red  to  yellow  or  brown 

variety  of  chalcedony  colored 

by  iron  oxids. 
Jolly   balance.     A   specific    gravity 

balance  in  which  use  is  made  of 

a  spiral  brass  wire. 

K 

Kainite,  331 

Kaolin.     Impure  kaolinite. 

Kaolinite,  403 

Kaolinization.  The  process  by 
which  kaolinite  is  formed. 

Kidney  ore.  A  reniform  variety  of 
hematite  from  England. 

Kunzite.  A  transparent  lilac- 
colored  variety  of  spodumene 
used  as  a  gem. 

Kyanite,  382 


Labradorite,  352 

Laccolith,  426 

Lamellar,  128 

Lampadite.     A  cupriferous  variety 

of  psilomelane. 
Lapilli,  439 
Lapis  lazuli.     A  mixture  of  lazurite, 

calcite,  diopside,  etc. 
Latite,  432 
Law    of    constancy    of    interfacial 

angles,  64 

of  rational  indices,  74 

of    rational    symmetric    inter- 
cepts, 74 
Lazurite,  357 
Lead     minerals.     Anglesite,     cerus- 

site,  galena,  Jamesonite,  mime- 

tite,  pyromorphite,  vanadinite, 

wulfenite. 

tests  for,  47 


516 


INDEX  AND  GLOSSARY 


Left-handed     quartz     crystal,     260 

Lenticular.    Lens-shaped. 

Lepidolite,  395 

Leucite,  355 

Light,  convergent,  193 

nature  of,  156 

polarized,  107 
Limburgite,  436 
Lime.     Calcium  oxid   (CaO),  often 

incorrectly    used    for    calcium 

carbonate. 
Limestone,  crystalline,  452 

oolitic,  448 

sedimentary,  442 
Limit  form,  79 
LIMONITE,  286 
Lindgren,  American  geologist  (1860- 

). 

Linear  projection,  88 
Lithia.     Lithium  oxid  (Li2O). 
Lithium  minerals.     Lepidolite,  spo- 

dumene. 

tests  for,  47 
Lodestone.     A  variety  of  magnetite 

which  acts  as  a  magnet. 
Luster,  154 

M 

Made.     Synonym  of  a  twin-crystal. 

Macroscopic.  The  same  as 
megascopic. 

Magma,  424 

Magnesia.     Magnesium  oxid  (MgO) 

Magnesian  limestone.  Limestones 
containing  magnesium,  but  in 
quantities  insufficient  for  dolo- 
mite. 

Magnesite,  298 

Magnesium  minerals.  Antigorite, 
anthophyllite,  brucite,  carnal- 
lite,  chondrodite,  chrysotile, 
dolomite,  enstatite,  forsterite, 


hydromagnesite,  kainite,  mag- 
nesite,  olivine,  spinel,  talc, 
tremolite,  and  other  silicates, 
tests  for,  47 

Magnetic  iron  ore.  A  synonym  of 
magnetite. 

pyrites.  A  synonym  of  pyr- 
rhotite. 

MAGNETITE,  279 

MALACHITE,  307 

Malleable.  Capable  of  being  ham- 
mered out  flat. 

Maltha,  414 

Mammillary,  128 

Manebach  twin,  345 

Manganese    minerals.     Franklinite, 
hausmannite,  manganite,  pyro- 
lusite,  psilomelane,    rhodochro- 
site,    rhodonite, 
tests  for,  47 

Manganite,  285 

Marble.  Any  limestone  that  will 
take  a  good  polish,  but  also  used 
by  some  for  a  metamorphic 
limestone. 

Marcasite,  239 

Martite.  A  pseudomorph  of  hema- 
tite (or  turyite)  after  magnetite. 

Massive.  Without  definite  form  or 
structure. 

Measurement,  of  axial  angle,  199 
of  crystals,  56 
of  index  of  refraction,  161 
of  interfacial  angles,  56 

Mechanically  formed  sedimentary 
rocks,  439 

Mediosilicic    igneous    rocks,    423 

Megascopic.  Capable  of  being  seen 
with  the  unaided  eye  in  contrast 
with  microscopic. 

Melanite.  A  titaniferous  variety  of 
andradite. 


INDEX  AND  GLOSSARY 


517 


Menaccanite.  A  synonym  of 
ilmenite. 

Mercury  minerals.     Cinnabar, 
tests  for,  47 

Metacolloids.  Microcrystalline  sub- 
stances of  colloidal  origin. 

Metamorphic  rocks,  450 

Metamorphism,  450 

Metasilicates,  343 

Metasomatic  replacement.  The  re- 
placement of  a  rock  mass  by 
solutions. 

Methylene  iodid,  150 

Miarolitic.  A  term  applied  to  the 
structure  of  injected  rocks  con- 
taining cavities,  lined  with 
euhedral  crystals. 

Micaceous,  128 

Mica  group,  392 
plate,  188 

Microchemical     gypsum     test,     43 

Microcline,  347 

Microcosmic  salt.  The  same  as  salt 
of  phosphorus  or  acid  sodium 
ammonium  phosphate  used  in 
blowpipe  analysis. 

Microlites.  Minute  crystals  found 
in  volcanic  glasses. 

Microperthite.  Perthite  on  a  small 
scale. 

Microscope,  polarizing,  172 

Miller,  W.  H.,  English  crystallo- 
grapher  and  mineralogist  (1801 
-1880). 

Miller  indices,  75 

Mimetic  twinning.  The  tendency  of 
twinning  to  raise  apparently  the 
grade  of  symmetry  and  thus 
imitate  other  crystals. 

Mimetite,  316 

Mineral,  definition  of,  1 

Mineraloid,  definition  of,  413 


Mispickel.     A  synonym  of  arseno- 

pyrite. 

Molecular  compounds,  10-11 
Molybdates,  338 
Molybdenite,  227 

Molybdenum  minerals.     Molybden- 
ite, wulfenite. 

tests  for,  48 

Monochromatic  light,  158 
Monoclinic  system,  115 
Monzonite,  431 
Moonstone.     A  variety  of  adularia 

with  a  pearly  reflection,  used  as 

a  gem. 
Morganite.     A  pink  variety  of  beryl, 

used  as  a  gem. 
Morphological  properties,  54 
Morphology,  crystal.     That  portion 

of  crystallography  which  deals 

with    the    form    and    internal 

structure  of  crystals. 
Mountain  cork  j  Varieties      of 

leather.  (        tremolite. 

Mundic.     A  local  name  for  pyrite 

or  marcasite. 
MUSCOVITE,  393 

N 

n.     A  symbol  used  for  the  index  of 

refraction. 
Native  copper,  221 

elements,  213 

gold,  218 

iron,  223 

platinum,  222 

silver,  220 
Natrolite,  411 
Negative  crystal.    A  cavity  within  a 

crystal  which  has  the  character- 
istic shape  of  the  crystal  itself. 

elongation,  189 

optically,  190 


518 


INDEX  AND  GLOSSARY 


Nepheline,  355 
Nepheline  syenite,  431 
Nephelite.   A  synonym  of  nepheline. 
Nephelite  syenite.     A  synonym  of 

nepheline  syenite. 

Nickel      minerals.      Garnierite, 
pentlandite. 

tests  for,  48 

Nicol.     The  same  as  a  Nicol  prism. 
Nicol  prism,  171 
Niobates,  312 
Niobium  minerals.     Columbite. 

tests  for,  48 

Niter.     Potassium  nitrate,  KNOs. 
Nitrates,  310 

tests  for,  48 
Nitratine,  322 
Nitric  acid  (reagent),  23 
Nodular,  128 
Non-metallic  luster.     Any  luster  but 

metallic. 
Nontronite.     The  iron  analogue  of 

kaolinite. 
Normal  salts,  10 
Norite,  434 
Notation  of  crystal  faces,  73 


0 


Oblique  extinction,  184 
Obsidian,  430 
Obtuse  bisectrix,  193 
Obtuse  rhombohedron,  105 
Occurrence  of  minerals,  417 
Ocher.     A    clay    colored    by    iron 

oxids. 

Octahedron,  90 
Ocular  of  microscope.     The  same  as 

the  eye-piece. 
Oligoclase,  351 
Oil  shale,  416 
OLIVINE,  373 


Olivine  gabbro,  435 

diabase.     A  diabase  containing 

olivine. 
Omphacite.     A  bright  green  variety 

of    diopside    characteristic     of 

eclogites. 
Onyx.     A    variety     of    chalcedony 

with  sharply  contrasted  bands 

of  colors. 
Onyx    marble.     A    banded    variety 

of  calcite  or  aragonite  formed 

by  water  solutions. 
Onyx,  Mexican.     The  same  as  onyx 

marble. 
Ooids.     The  individual  spheres  of  an 

oolitic  rock. 
Oolite.     A  rock  made  up  of  minute 

spheres  formed  by  concretion- 
ary action. 
Oolitic  limestone,  448 

structure.     The  structure  of  an 

oolite. 
OPAL,  262 
Opalescence.  The  peculiar  milky 

appearance  often  seen  in  opal. 
Opalized  wood.     A  replacement  of 

wood  by  opal. 
Open  form,  72 
Open  tube  tests,  27 
Ophicalcite,  455 
Optical  anomalies,  206 

character,  190 

constants,  192 

orientation,  207 

properties,  156 

tests,  204 
Optic  axes,  191 

axis.     The  c-axis  of  reference  in 
the   tetragonal   and   hexa- 
gonal systems. 
'ellipse,  189 

ellipsoid,  189 


INDEX  AND  GLOSSARY 


519 


Optic  normal,  191 

Optics,  crystal.  Optical  crystal- 
lography or  the  branch  of  crys- 
tallography that  deals  with  the 
transmission  of  light  in  crystals. 

Order  of  succession,  418 

Ordinary  ray,  169 

Ores,  456 

Organically-derived        sedimentary 
rocks,  442 

Oriental  amethyst.     A  purple  vari- 
ety of  corundum, 
emerald.     A   green   variety   of 

corundum, 
ruby.     The  true  ruby,  a  variety 

of  corundum, 
sapphire.     The  true  sapphire,  a 

variety  of  corundum, 
topaz.     A    yellow    variety    of 
corundum. 

Orientation  of  a  crystal.  The  plac- 
ing of  a  crystal  in  its  conven- 
tional position  with  the  c-axis 
vertical  and  the  o-axis  pointing 
toward  the  observer, 
optical,  207 

Origin  of  minerals,  417 

Ortho-axis.  The  fr-axis  in  the  mono- 
clinic  system. 

Orthographic  drawing  or  projection, 
84 

Orthorhombic  system,  111 

ORTHOCLASE,  344 

Orthosilicates,  343 

Oxidizing  flame,  25 

Oxids,  258 

Ozocerite,  415 


P.     Symbol     used     for     plane     of 

symmetry. 
Paragenesis,  418 


Parallel  growth,  123 

Parameters.  The  relative  intercepts 
of  the  unit  face  (1 1 1)  on  the  axes 
of  reference. 

Paramorph,  424 

Parting,  131 

Path  difference,  176 

Peacock  copper.  Tarnished  chal- 
copyrite. 

Pearl-spar.    A  synonym  of  dolomite. 

Pearly  luster,  154 

Pediad  class.  The  asymmetric  crys- 
tal class,  in  which  the  general 
form  is  a  pedion. 

Pedion,  69 

Pegmatites,  437 

Penetration  twin,  124 

Penfield.  American  mineralogist 
(1856-1906). 

Pentagonal  dodecahedron.  A  syn- 
onym of  pyritohedron. 

Pentlandite,  233 

Percussion-figure,  392 

Pericline.  A  variety  of  albite  elon- 
gated in  the  direction  of  the 
6-axis. 

Pericline  twin-law.  The  twin-law 
for  plagioclase  feldspars  in 
which  the  6-axis  is  the  twin 
axis,  349 

Peridot.     A  gem  variety  of  olivine. 

Peridotite,  436 

Perlite,  430 

Persilicic  igneous  rocks,  423 

Perthite.  An  intergrowth  of  micro- 
cline  and  albite,  348 

Petrifaction.  The  replacement  of 
fossils  by  mineral  substances,  421 

Petrographic  microscope,  172 

Petrography.  The  science  which 
treats  of  rocks,  especially  from 
the  descriptive  side. 


520 


INDEX  AND  GLOSSARY 


Petroleum,  414 

Petrology.  The  study  of  rocks 
from  a  broad  geological  stand- 
point. 

Phantom  crystal.  A  crystal  in 
which  an  earlier  stage  of  growth 
is  marked  by  a  color  difference 
or  a  row  of  inclusions. 

Phase,  158 

Phenocryst,  425 

Phlogopite,  397 

Phonolite,  432 

Phosphates,  310 
tests  for,  49 

Phosphate  rock.  A  synonym  of 
phosphorite. 

Phosphorite,  450 

Physical  properties,  147 

Picotite.  A  chromium-bearing  va- 
riety of  spinel. 

Piezoelectric.  The  property  of  de- 
veloping electricity  by  changes 
in  pressure  exerted  upon  a 
crystal. 

Pinacoid,  69 

Pinacoidal  class,  121 

Pisolitic.  Made  up  of  spheres  the 
size  of  buck-shot  or  larger  which 
have  been  formed  by  concre- 
tionary action. 

Pistazite.     A  synonym  of  epidote. 

Pitchblende,  324 

Pitchstone,  430 

PLAGIOCLASE,  348 

Plaster  tablets,  21 
tests  on,  30 

Platinum,  222 
tests  for,  49 

Pleochroism,  201 

Pleonaste.  An  iron-bearing  variety 
of  spinel. 

Plumose.     Feather-like. 


Plutonic  rocks.  Deep-seated  igne- 
ous rocks  as  contrasted  with  the 
volcanic  or  surface  igneous 
rocks. 

Point-systems,  138 

Polar  axis.  An  axis  of  symmetry 
which  has  different  faces  at 
opposite  ends. 

edges.  Edges  that  intersect 
the  c-axis. 

Polariscope,  193 

Polarized  light,  167 

Polarizer,  173 

Polarizing  microscope,  172 

Polybasite,  249 

Polymorphism,  18 

Polysilicates,  343 

Polysynthetic  twin,  236 

Porodine,  17 

Porphyritic,  425 

Porphyry.  An  igneous  rock  with  a 
porphyritic  texture. 

Positive  elongation,  189 
optically,  190 

Potash  feldspar.  Either  orthoclase 
or  microcline. 

Potassa.     Potassium  oxid  (K2O). 

Potassium  minerals.  Adularia,  alu- 
nite,  biotite,  carnallite,  jarosite, 
kainite,  lepidolite,leucite,  micro- 
cline, muscovite,  orthoclase,  syl- 
vite. 
tests  for,  49 

Precious  opal.  A  variety  of  opal 
with  a  play  of  colors  and  used  as 
a  gem. 

stones.  Diamond,  emerald, 
ruby,  and  sapphire.  Other 
stones  used  as  gems  are  called 
semi-precious. 

Prehnite,  387 

Primary,  460 


INDEX  AND  GLOSSARY 


521 


Primitive  form.  A  crystal  form 
from  which  other  forms  may  be 
derived. 

Principal  axis.  The  c-axis  of  refer- 
ence (A3,  A4,  or  A6)  in  the 
tetragonal  and  hexagonal  sys- 
tems. 

Prism,  69 

Prismatic  class,  116 
habit,  469 

Projection,  clinographic,  85 
linear,  88 
orthographic,  84 

Proustite.  The  arsenic  analogue  of 
pyrargyrite. 

Pseudohexagonal.  Orthorhombic 
or  monoclinic  crystals  which 
simulate  crystals  of  the  hexa- 
gonal system. 

Pseudomorph,  421 

PSILOMELANE,  289 

Pumice,  430 

Pycnometer,  150 

Pyramid,  69 

Pyramidal  habit,  469 

Pyrargyrite,  246 

PYRITE,  236 

Pyrites.  A  synonym  of  pyrite. 

Pyritohedron,  95 

Pyroclastic  rocks,  438 

Pyroelectric,  389 

Pyrognostic  tests.     Blowpipe  tests, 
25 

Pyrolusite,  274 

Pyromorphite,  315 

Pyrope,  371 

Pyrophyllite,  401 

PYROXENE,  359 
group,  357 

Pyroxenite,  436 

PYRRHOTITE,  235 


Q 


Qualitative  scheme,  39 

Quarter-undulation  mica  plate,  188 

QUARTZ,  258 
wedge,  199 

Quartzite,  452 

Quartz  monzonite.     A  quartz-  bear- 
ing monzonite. 

porphyry.  An  altered,  devitri- 
fied  rhyolite  or  rhyolite  por- 
phyry. 


R 


R.     A  general  symbol  standing  for 

some  metal. 

Radical.     A  group  of  chemical  ele- 
ments which  act  as  a  unit  such 

as  NH4  or  SO4. 
Rare-earth  metals.    Cerium,  erbium, 

lanthanum,  neodidymium,  pras- 

codidymium,      thorium,      and 

yttrium. 

Rational  indices,  74 
Ray  of  light,  158 
Reagents,  22-25 
Reduction  color  tests,  33 
Reducing  flame,  26 
Reflection  goniometer,  57 

total,  161 

twins,  123 
Refraction  of  light,  159 

index  of,  159 

indices  of,  table  of,  207-209 
Refractometer,  163 
Regional  metamorphism,  451 
Regular  system.     The  same  as  the 

isometric  system. 
Relief,  167 
Reniform,  128 


522 


INDEX  AND  GLOSSARY 


Replacement,  420 

Residual  minerals,  440 

Resinous.     The  luster  of  resin. 

Resins,  414 

Reticulate.    Made  up  of  a  net-work. 

Rhodochrosite,  300 

Rhodonite,  362 

Rhombic  bipyramid,  69 

bipyramidal  class,  112 

bisphenoid,  71 

bisphenoidal  class,  80 

dodecahedron,  90 

prism,  69 

prismatic  class.     The   same   as 
the  prismatic  class. 

pyramid,  69 

pyramidal  class,  80 

section  of  plagioclase  crystals, 

350 
Rhombohedral  carbonates,  290-301 

class,  80 

subsystem,  102 
Rhombohedron,  71 
Rhyolite,  430 

porphyry,  430 

Right-handed  quartz  crystal,  260 
Rock     crystal.     The     clear     trans- 
parent variety  of  quartz  used 

for  ornamental  purposes  and  in 

optical  and  piezoelectric  work. 
Rocks,  422 

,  igneous,  422 

,  metamorphic,  450 

,  sedimentary,  439 
Rock-forming  minerals,  424 
Rock-salt,  446 
Rotation  twins,  123 
Rotatory-reflection.         The   simul- 
taneous operations  of  reflection 

and  rotation. 
Rubellite.     The  pink  to  red  variety 

of  tourmaline. 


Ruby.     The  transparent  red  variety 

of  corundum. 

copper.     A  synonym  of  cuprite. 

silver.     A  group  name  for  pyr- 

argyrite  and  proustite. 
Rutile,  273 


S 


Saline  residues,  447 

Salt.  (1)  Synonym  of  halite.  (2) 
compounds  formed  by  the  union 
of  bases  with  acids. 

Salt  of  phosphorus.  Hydrous  acid 
sodium  ammonium  phosphate 
used  in  bead  tests. 

Salts,  acid,  10 
basic,  10 
normal,  10 

Sand,  440 

Sandstone,  441 

Sanidine.  A  clear  transparent  vari- 
ety of  orthoclase  found  in  vol- 
canic igneous  rocks. 

Sapphire.  The  transparent  blue 
variety  of  corundum. 

Sard.  A  brownish-red  variety  of 
chalcedony  used  as  a  gem. 

Sardonyx.  Agate  with  red  and 
white  bands. 

Satin-spar.  A  fibrous  variety  of 
gypsum. 

Scalar,  147 

Scalenohedral,  hexagonal,  class,  104 
tetragonal,  class,  80 

Scalenohedron.  There  are  two 
kinds  of  scalenohedrons  but 
when  no  qualifying  term  is  used 
the  hexagonal  scalenohedron  is 
meant. 

Scalenohedron,  hexagonal,  71 
tetragonal,  71 


INDEX  AND  GLOSSARY 


523 


Scale  of  fusibility,  34 

hardness,  153 
Scapolite,  377 
Scheelite,  339 
Schistose.     With  the  characters  of  a 

schist. 
Schists,  451 
Schoenflies,  German  mathematician 

(1853 ). 

Schorl.    An  old  name  for  tourmaline. 
Sclerometer,  154 
Screw-axis  of  symmetry,  138 
Secondary,  460 

enrichment,  460 
Sectile.     Capable  of  being  cut  by  a 

knife  but  not  flattened  out  by  a 

hammer. 

Sedimentary  rocks,  439 
Selenite.     The  cleavable  variety  of 

gypsum. 
Selenite  plate.     A  plate  of  selenite 

showing  the  sensitive  tint  (q.v.}. 
Semi-opal.     Common   opal   as   dis- 
tinguished from  precious  opal 

and  fire  opal. 

Sensitive   tint.     The   purple    inter- 
ference color  between  red  of  the 

first-order     and     blue     of    the 

second-order,  185 
Sericite,  394 
Serpentine  (rock),  455 

minerals       of.  Antigorite, 

chrysotile. 
Serpentinization.     The  alteration  of 

peridotite    to    serpentine. 
Shade,  155 
Shale,  442 
SIDERITE,  299 
Silica  minerals,  stability  relations  of, 

265 

Silicates,  341 
Siliceous  sinter,  449 


Sill,  426 
Sillimanite,  383 
SILVER,  220 

assay,  34 

cupellation,  34 

glance.     A         synonym          of 
argentite. 

minerals  of.  Argentite,  cerar- 
gyrite,  polybasite,  pyrargy- 
rite,  silver,  stephanite. 

tests  for,  50 
Skeleton    crystals.     More    or    less 

hollow  crystals  formed  by  rapid 

crystallization. 
Slate,  451 
Slower  ray,  187 
Smaltite,  238 
SMITHSONITE,  301 
Smoky  quartz.     A  variety  of  quartz 

with  a  brown  pigment. 
Snow,  266 
Soapstone.     A    massive    metamor- 

phic  rock  consisting  essentially 

of  talc. 
Soda.     Sodium  oxid  (Na2O).     This 

term  is  often  incorrectly  used 

for  sodium  carbonate. 
Soda-lime  feldspar.     A  synonym  for 

plagioclase. 
Sodalite,  356 

Soda  niter.     A  synonym  of  nitratine. 
Sodium    carbonate    bead    tests,  33 

metaphosphate  bead  tests,  32 

minerals.  Albite,  analcite,  cry- 
olite, glaucophane,  halite, 
lazurite,  .natrolite,  nepheline, 
nitratine,  sodalite,  ulexite, 
scapolite. 

tests  for,  50 
Sohncke,     German     mathematician 

(1842 ). 

Solid  solutions,  16 


524 


INDEX  AND  GLOSSARY 


Solubility  of  minerals,  38 

Solutions,  solid,  16 

Space-groups,  139 

Space-lattice,  132 

Specific  gravity,  148 

Specific  gravity  balance,  149 

Specular  iron-ore.  A  synonym  of 
hematite. 

SPHALERITE,  231 

Spherulites,  128 

Sphene.     A  synonym  of  titanite. 

Sphenoid,  69 

Sphenoidal  class,  80 

Spinel,  278 

Spodumene,  370 

Stalactitic,  128 

Staurolite,  387 

Steatite.     The  same  as  soapstone. 

Stellate.     With  a  radiate  star  effect. 

Steno.  A  Danish  scientist,  who  first 
discovered  the  law  of  constancy 
of  interfacial  angles  (1638- 
1687). 

Stephanite,  248 

Stibiconite,  275 

STIBNITE,  225 

Stilbite,  409 

Streak,  155 

Streak-plate,  155 

Stream-tin.  A  variety  of  cassiterite 
found  in  placer  deposits. 

Striations,  oscillatory.  Striations 
produced  by  alternate  develop- 
ment of  two  adjacent  forms, 
twinning,  125 

Strontianite,  304 

Strontium,   minerals  of.     Celestite, 
strontianite. 
tests  for,  51 

Structure,  internal,  of  crystals,  132 
of  minerals,  128 
of  igneous  rocks,  425 


Sub-.     A  prefix  indicating  a  lower 

quality    or    degree    than    the 

normal. 
Subhedral,  54 
Subjacent,  426 
Sublimates  on  charcoal,  29 

in  closed  tube,  28 

in  open  tube,  27 

on  plaster,  30 

Submetallic.     With  a  luster  inter- 
mediate between  metallic  and 

adamantine. 
Subsectile.     Imperfectly  sectile  like 

chalcocite. 
Subsilicates.     Silicates  of  the  type 

n  RO-SiO2  where  n  >  2,  343 
Subsilicic,  423 

Succession,  order  of,  of  minerals,  418 
Sulfantimonates,  243 
Sulfantimonites,  243 
Sulfarsenates,  243 
Sulfarsenites,  243 
Sulfates,  326 

tests  for,  51 
Sulfids,  225 

tests  for,  51 
Sulfo-acids,  9 
Sulfoferrites,  243 
Sulfo-salts,  243 
SULFUR,  217 
Sulfuric  acid  (reagent),  23 
Sunstone.     A  variety  of  oligoclase 

with  a  spangled  appearance  due 

to    inclusions    of    hematite    or 

goethite. 
Supergene,  461 

enrichment,  460 
Surficial.     Deposits    found    on    the 

immediate  surface  of  the  earth. 
Syenite,  431 

porphyry,  431 
Sylvite,  253 


INDEX  AND  GLOSSARY 


525 


Symbols  of  crystal  faces,  74 
Symmetry,  60 
Symmetry,  axis  of,  60 

center  of,  62 

composite  62 

plane  of,  61 

Symmetrical  extinction,  184 
Synthesis  of  minerals,  418 
Systems,  crystal,  81 


Table,  of  the  32  crystal  classes,  80 

of  atomic  weights,  6-7 
Tables,  determinative,  466 

indices  of  refraction,  207-209 

specific  gravity,  150-152 
Tabular  habit,  468 
TALC,  400 
Talc  schist,  451 
Tarnish.     A  thin  surface  film  caused 

by  oxidation. 
Tellurids,  242 
Tellurium,  minerals  of.     Calaverite. 

tests  for,  52 
Test-plates,  188 
Tetartohedral,  83 

Tetartoid.     The   twelve-faced    gen- 
eral form  of  the  tetartoidal  class. 
Tetartoidal  class,  80 
Tetragonal    bipyramidal    class,    80 

bisphenoidal  class,  80 

pyramidal  class,  80 

scalenohedral  class,  80 

system,  97 

trapezohedral  class,  80 
TETRAHEDRITE,  247 
Tetrahedron,  94 
Tetrahexahedron,  91 
Texture  of  igneous  rocks,  424 
Thin-section.     A     paper-thin     slice 

of  a  mineral  or  rock,  used  for 


microscopic    examination    and 

optical  tests. 
Tin  minerals.     Cassiterite. 

tests  for,  52 
Tin-stone.     A     synonym     of     cas- 

siterite. 

Tin-stone  veins,  457 
Tint,  155 
Titanite,  412 
Titanium  minerals.    Ilmenite,  rutile, 

titanite. 

tests  for,  52 
Topaz,  380 
Total  reflection,  161 
Touchstone.     A    black    variety    of 

chalcedony  used  for  testing  the 

purity  of  gold  and  other  metals. 

The  color  and  solubility  of  the 

streak  are  tested. 
TOURMALINE,  388 
Trachyte,  431 
Trachyte  porphyry 
Translation,  136 
Trap.     A  dense  black  igneous  rock, 

usually  basalt  or  diabase. 
Trapezohedron,  91 

hexagonal,  71 

tetragonal,  71 

trigonal,  71 
Travertine,  448 
Tremolite,  364 
Trichroic,  202 
Triclinic  system,  121 
Tridymite,  263 
Trigonal  bipyramid,  70 

bipyramidal  class,  80 

prism,  68 

pyramid,  70 

pyramidal  class,  80 

trapezohedron,  71 

trapezohedral  class,  109 
Trimorphism.     The  particular  case 


526 


INDEX  AND  GLOSSARY 


of  polymorphism  in  which  there 
are  three  polymorphs. 

Trisoctahedron,  91 

Tristetrahedron,  94 

Troostite.  A  manganese-bearing 
variety  of  willemite. 

Truncation.  The  modification  of  an 
edge  of  a  crystal  by  a  face  that 
makes  equal  angles  with  adja- 
cent faces. 

Tufa,  calcareous,  448 

Tuff,  439 

Turgite.     The  same  as  turyite. 

Tungstates,  338 

Tungsten  minerals.     Scheelite,  wol- 
framite, 
tests  for,  53 

Turkey-fat  ore.  A  yellow  variety 
of  smithsonite  containing  CdS 
in  solid  solution. 

Turmeric  paper.  Paper  saturated 
with  a  solution  of  turmeric  and 
used  for  testing  borates  and 
zirconium. 

Turquois,  320 

Turyite,  271 

Twin-axis,  123 
-crystals,  123 
-law,  124 
-plane,  123 
-seam,  125 

Twinning  striations,  125 

Type  symbols,  75 

Types  chemical,  10 

U 

Ulexite,  324 
Ultrabasic  rocks,  437 
Uniaxial,  190 

Unisilicates.     The   same   as   ortho- 
silicates.     Rn2SiO4  = 
2R"OSiO2,  343 


Unit  bipyramid.     The  form  {ill) 
or  {1011}. 
face,  75 
forms.      The     forms      (llOj, 

(101),  (Oil),  and  fill), 
prism.     The  form  jllO). 
pyramid.     The  form    Jill)   or 

lion), 
rhombohedron.     The    rhombo- 

hedron,  (lOll). 
series  of  bipyramids,  113 

Uralite.  An  actinolite  pseudo- 
morph  after,  or  alteration  of, 
pyroxene. 

Uralitization.  The  alteration  in- 
volved in  the  formation  of 
uralite. 

Uraninite.  The  crystalline  equiva- 
lent of  pitchblende. 

Uranium  minerals.    Carnotite,  pitch- 
blende, 
tests  for,  53 


Valencianite.  A  synonym  of 
adularia. 

Vanadates,  317,  321 

Vanadinite,  317 

Vanadium    minerals.     Carnotite 
vanadinite. 
tests  for,  53 

Variations  in  the  chemical  composi- 
tion of  minerals,  4 

Vectorial,  147 

Veins,  455 

minerals  of,  456 

Verd  antique.  A  serpentine  rock 
veined  or  mottled  with  car- 
bonates. 

Vertices  of  a  crystal,  55 

Vesicular,  425 

Vesuvianite,  378 


INDEX  AND  GLOSSARY 


527 


Vibration  directions,  184 

plane  of,  167 
Vicinal    faces.     Crystal   faces   with 

high    indices   which    adjoin    or 

replace  faces  with  lower  indices. 
Vitreous  luster,  154 
Vitrophyre,  430 
Vivianite,  318 
Volcanic  ash,  439 

bombs,  439 

breccia.     Practically  the  same 
as  an  agglomerate. 

dust,  439 

emanations;  437 

rocks,  426 

tuff,  439 
Vug.     A  cavity  in  a  vein  lined  with 

crystals. 

W 

Water  of  constitution,  11 

of  crystallization,  11 

tests  for,  46 
Wave-front,  158 

-length,  158 

-motion,  156 
Weathering,  440 
Wedge,  gypsum,  206 

quartz,  199 
Weiss.     German    crystallographer 

(1780-1856). 
We'ss  symbols,  75 
Wernerite.     The     most     prominent 

member  of  the  scapolite  group. 


Whewellite.  Naturally  occurring 
hydrous  calcium  oxalate,  416 

Willemite,  375 

Witherite,  305 

Wolframite,  338 

Wollaston.  An  English  chemist 
who  invented  the  reflection 
goniometer  (1766  -  1828). 

Wollastonite,  369 

Wood-opal.  The  same  as  opalized 
wood. 

Wood,  silicified.  Wood  replaced  by 
chalcedony,  opal,  or  quartz. 

Wulfenite,  340 


X-rays  and  crystal  structure,  132 
X-ray  spectrometer,  141 


ZEOLITE  (group),  407 

Zinc  blende.     A  synonym  of  sphaler- 
ite. 

minerals.     Calamine,    franklin- 
ite,    smithsonite,    sphalerite, 
willemite. 
tests  for,  53 

Zircon,  379 

Zirconium    minerals.     Zircon, 
tests  for,  53 

Zone  of  crystal  faces,  56 

Zone  of  oxidation,  459 

Zone  of  secondary  enrichment,  460 


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